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I'm of the opinion that a story should stand on its own; any worldbuilding that is necessary for the plot and story to function should be part of the story. However, at the same time, the world I write in is an extensive, fully-realised world which has lots of little details which, if taken note of, give the story a little bit of extra context.

The story I'm writing covers a woman born and raised in a mage cult. Her understanding of the world is considerably limited, racist, and toxic; her entire journey is about shaking these shackles and becoming a better person.

Everything strictly necessary for the story will be included; the outwardly animalistic and non-magical nature of orcs that causes the protagonist's cult (and therefore the protag) to be racist against them, the cult's racial hierarchy, how magic works, the protag's backstory of being crap at any remotely covert work due to killing a nobleman the first and only time she attempted it, et cetera, et cetera.

However, there's plenty of stuff which, while obliquely referenced, aren't strictly expanded upon, merely remarked upon as part of the world. 'The Crown' or 'the Royal Electorate' as an entity that rules the kingdom the cult is within, the Houses of Sinhelios (the noble family who the protagonist stabbed the heir of as part of her backstory) and Sandspark (a goblinese family mentioned as being liege lords of the Ground's Scar Mining Colony, an area investigated by the cult), mentions of cities such as Godswater and Parakos, et cetera.

My question is, for the sake of satisfying reader interest, would it be worthwhile writing an appendix summarising certain inessential worldbuilding entities that's entirely optional for a reader to peruse?

I think an analogous example would be how George RR Martin does it in A Song of Ice and Fire; if I recall correctly, A Dance with Dragons has summaries of the Houses Frey, Florent, Hightower, and Velaryon, all houses that, while their specifics may become important later, aren't particularly elaborated upon or are strictly necessary in the story thus far.

My question is, for the sake of satisfying reader interest, would it be worthwhile writing an appendix summarising certain inessential worldbuilding entities that's entirely optional for a reader to peruse?

It can be worthwhile. Some readers are "hungry" after a story ends, and will devour any appendix you provide. It can be argued that since you have that material sketched out, it's worth to make it available to the audience.

Yet, I'd like to leave here a couple of warnings:

• 
  Don't get self-indulgent. There's a fine line between adding information for the more curious readers, and doing so just to stroke one's own ego. Maybe the audience would like to know something more about the city of Godswater, true enough; but no matter how smart your worldbuilding is, they probably don't want to read about the entire history of its government.


• 
  Appendix writing can get you worldbuilding disease: after all, there will be always points to expand. Following the previous example, if you included an excerpt about Godswater, you'll be tempted to put in a similar one for each city that has been referenced in the novel. But is it really worth it? Borrowing the famouse iceberg metaphor, most of the worldbuilding is doomed to remain submerged anyway. Either that or you'd be dumping exposition.


• 
  Ideas are - sometimes - like bullets, in the sense you don't want to fire them all at once without taking aim.

I'm going to expand on the last point. You have completed a story from a PoV that explores just a bit of the world you have created; you find yourself with "additional material" that the readers won't get to enjoy. Sure, you can use that material to make appendices, but should you really?

Any good idea, and the good work, that you have put into worldbuilding would be better showcased in an actual story. You could write another novel, or a set of short stories. Instead of compiling a dictionary entry about the city of Godswater, you could write something in there. This would be good both marketing-wise and well, writing-wise. Readers who loved your first novel are more likely to be interested in a second, even losely related one.

Sure, writing an appendix is not "wasting a bullet". At least not necessarily. But if you're in the lucky situation of your readers wanting more of your work, maybe short online stories would serve you better than appendices.

To add a bullet point to Liquid's excellent list:

Creating an appendix permanently locks your worldbuilding

As long as your worldbuilding only exists in your mind and your notes, you are free to change and adjust it however much you need to. As soon as you publish it it becomes locked in stone, and altering it will come with costs in reader confusion and disengagement.

If you plan to write further in the world that you've built, caution should be taken to leave yourself flexibility in the worldbuilding to expand into new stories.

One way you can dodge this problem is by creating "in world" appendices. There is some expectation that an appendix written by the author accurately depicts the world. But an appendix written by Bob the Sorcerer can be full of lies, misstatements, and inaccuracies. It gives you room to maneuver, while also being more interesting for readers.

Also consider that if you are trying to build an internet presence, worldbuilding is an excellent thing to put on your website in order to attract attention to it and hold interest. And a website is more dynamic than a book, so if you need to adjust a few details than you can.

An appendix such as a glossary, a geographic map, or a timeline is especially helpful when you have a collection of work in the same world. In the ideal, it is a quick way for a reader to see the relationship between works.

Although each of your works can stand alone, it may be helpful for an avid follower of your world to know how things fit together. If I pick up a book with the title "The Little Mermaid versus Hitler", I will know it is WW2 era, probably set in Denmark during the German occupation. The title "Stephanie versus Glisdhum" doesn't carry the same clear expectations.

I have found a glossary and geographic reference handy in some books. Consider the map I found helpful in "The Lord of the Rings". OTOH, the Elfish dictionary exceeded my need, so I don't read it.

For the appendix in this book, I would limit it to a few details that add spark to this book, and hint details you will expand in future books. Don't tell the story in the appendix, but create an expectation that this book is set in an organic context from which many more stories will grow.

I have a technique I call the "Scruffy Nerf Herder Test". If I'm using something totally not real, I need to make sure that the context in the dialog makes the meaning of the word perfectly clear. The name of course comes from the dialog between Princess Leia and Han Solo in Star Wars where Leia hurls the line "Why you stuck up, half-witted, scruffy looking Nerf Herder!" and storms out of the room, with Han responding by repeating what he perceives to be the worst insult: "Scruffy Looking?!"

The gag is an excellent example as to how to world build without needing to detail. From Leia's dialog, it's clear that she's throwing together insults haphazardly and whatever a Nerf is, it's a gross animal. I know the author has described a Nerf outside of the film, I do not care... whatever a Nerf is, I know shepherding a herd of them means somewhere in my past, I should have listened to that hooded fellow who was telling me I should go home and rethink my life.

Not knowing what Nerf is also does not hurt the joke: Of the three insults I do know, Han is taking offense at the tamest one of them all, Scruffy looking. This is a typical comedy gag where the least offensive insult is the one that gets the most protests from the person on the receiving end. It's as if Han was saying, "Yeah, I maybe stuck up and half-witted... and yeah, I smuggled some live stock back in the day, who hasn't? But Scruffy Looking?! Why, I never?!"

Alternatively, we know Han has just moments earlier, hit on Leia and now she is flummoxed and put in an awkward situation... So she comes up with a insults that are lacking for punch and Han is reacting in this way to mock her use of the weak Scruffy-looking as a cutting insult. Either way, Han and Leia are bickering despite having resisted feelings for each other, playing the "Fighting like and Old Married Couple" trope common in romantic fictions.

Either way, the test is that, if the implication of the dialog can carry all that is needed by the dialog, even with some of the concepts not being known to the reader. For example, your Godswater discussion can be easily carried by refering to it as the "City of Godswater" and maybe speaking to it's characterization in the Universe (A wretched Hive of Scum and Villainy? A shining beacon on a hill? Have the character give a passing nod to the outsider's perspective and move along... save it for the sequel. Alternatively, as this is a Fantasy, the Fantasy world map is a pretty common occurrence, you can mark Godswater on the map and move on. Even in Avatar the entire World Map is revealed in the first Episode so one can see locations with respect to their journey... there are even maps that trace the Journey around the world.

The crown or royal electorate automatically reek of government offices (especially a distant one as there is no mention of the Monarch by name. There are even some countries that still exist in current year that use terms like this. For example, England often uses "The Crown" to speak of the generic Monarch or something belonging to her.

For the politics between families of note, you can have a little blurb of antagonism that hints there's a past history of unpleasantness but not a necessary overt or espoused reason for the antagonism. Or have the character say something like "Last time that happened, you nearly died" and when the Protagonists new friend asks why the guy almost died the last time that happen, have the protagonist casually drop "Cause I stabbed him." The Goblin family can easily be discussed by mentioning it's "The Goblin Clan/Family/Whatever group goblins live in [Family Name]" to indicate that they are a family and they are goblins.

Even if you do have a lot of world building concepts, you shouldn't need an appendex on your book to get the point across. Show, don't tell, remember? Keep in mind that in Harry Potter, we waited for seven books to learn that Dumbledoor's light dampening "Put Outer" was really called a "Dilluminator".