(Note: Originally this post was going to be part of my spoilery analysis for Gideon the Ninth, word-for-word. But it occurred to me that this part of the analysis didn't actually contain any major spoilers for Gideon, and it would serve well as its own standalone post, so that's what we're doing here.)
I don't read a lot of spec-fic, but the impression I've gotten from it is that it's all about the world-building. Some readers seek this for the escapist aspects, but for the most part, I think there's just a certain kind of person who enjoys thinking through how all the different parts of the world interact with and impact each other, and part of the way they think about this is by examining hypothetical worlds where just a few of the variables are changed. I think this kind of thing must be of special interest to those who study fields related to history or anthropology, Lord of the Rings being an obvious example here.
However, since I prefer to view the world primarily through the lens of interpersonal relationships, worldbuilding isn't something that's inherently appealing to me personally. It's a contributing factor to why GtN just isn't my kind of book, and why I'd usually rather be reading romances.
Still, I'm willing to learn from a story that focuses on different aspects of a story than what I usually focus on. It might help me flesh out aspects of my own stories that I'd otherwise overlook, even aspects that would be supportive of the parts I am more interested in. So let's think it over for a bit.
Creating inspiration in readers
If there's one thing I do admire about stories that focus on worldbuilding, it's how they have the potential to inspire many other people to be creative. If you think of any of the big fanbases out there, the ones who create fanfics and fanart and fansongs and fanvids and every other type of fanwork you can think of, usually the property in question is something that has a big focus on worldbuilding.
I feel that the reason for this is because, inherently, if something has well-established worldbuilding, it is providing a sandbox for you to play in. If something has adequately fluid worldbuilding, anyone with any creative inclinations at all can find a way to make a fanwork that is representative of their own creative strengths and values while also still feeling like it could easily be part of the world that it is representative of, because a properly-constructed world will be open to all sorts of different people living and creating spaces within it.
Though, this does raise the question of how such a world is constructed. Not every fantasy story results in such an inspired fanbase, after all.
This is something that was already on my mind before I read GtN, but I reflected on what I thought GtN got right with it, and I think a big part of it is just clearly establishing the broad aesthetic themes of the world. In GtN's case, the biggest theme is "necromancy". This, in turn, expands into broader aesthetics of darkness, murkiness, metal, chains, and of course lots of skeletons and other imagery of death and decay. These broader aesthetic themes are frequently reinforced with descriptions of every new location that the characters explore. (These themes are reiterated by the world's culture, as well, and that matters a lot for the sake of consistency, but I think the visuals are what help reinforce it in the most concrete ways.)
If the aesthetics are broken down into easily-summarizable themes like this--and if these themes are reinforced often enough that there's no way people will forget them--then it also becomes easy for anyone else to add something to the work that is thematically appropriate to it while still adding some other elements to it that are unique to them. It allows other people to come in and expand the world, in a way that feels like it all still naturally connects to the parts that already exist.
Customizing a world to your own tastes
With the book's focus on necromancy, I think my initial assumption before actually reading GtN was that it was going to be a fantasy story, specifically. But generally, "fantasy" brings implications of, you know, medieval setting, orcs and elves, all the usual LotR/D&D stuff. And, well, GtN does have swords and sorcery, and cavaliers protecting what are effectively monarchical rulers.
But it also has space travel as a common element of life, which is definitely more of a sci-fi thing. The specific way in which necromancy functions in this universe has some sci-fi vibes as well, with new magic being referred to as "theorems" and the undead following what is referred to as "programming". And this isn't getting into the aspects of the story and setting that take on a more contemporary or historical feel. It brings an aspect of anachronism to the setting, making it feel alien in a timeless kind of way.
Now, there is something to be said for sticking to a particular group of tropes and expectations when developing a setting. A sense of familiarity can always help ease a reader into what they're reading, allow them to fill in a lot of the blanks of the background so that you can go ahead and jump directly to the more specific details of your setting.
However, there is also a certain power that comes from being willing to break these constraints and pick and choose different tropes from different styles of stories in order to cobble together something brand new for your story. It requires more effort, yes, and it means hoping the reader is willing to feel a little lost for a longer period of time while they get used to the particulars of the setting. But the end result is something more memorable, and, if handled properly, something that is more strongly reinforcing of those "core themes of the setting" that I described in the previous section, the ones that are so inspiring to the fans.
But aside from that, I also can't help but suspect that it's more fun for the author, too. I mean, I suppose it depends on how much love an author has for specifically fantasy or specifically sci-fi settings as they already exist, how much an author wants to make their own "fanfics" of those pre-existing general settings. But for people like me, the idea of creating a world that is unique to my personal tastes, a world that prioritizes vibes over logic, a world that redefines the lines upon which "consistency" is drawn, is much more appealing.
Hidden tutorials
In video game design, games are often praised for teaching the player the mechanics of the game without the player even realizing that it's happening.
Let's use Super Metroid as an example. In the beginning of the game, the player's first task is exploring an abandoned space station. Initially, there are no threats to encounter, so the player is free to learn the basics of movement without worry; and because they are investigating the mystery of what happened on the station, they are inspired to do so on their own. The player learns to shoot, because doing so is required to open the doors. Eventually, a mock battle takes place where the player gets a taste of the kinds of fighting they'll eventually need to do, but the enemy runs off before the player can run out of health. The player is then given their first real test by being required to leave the space station on a time limit, but it's through a linear area that they have already taken as much time as they need to become comfortable with.
You might be wondering what this has to do with writing. Well, the point is that well-designed video games can find ways to teach the player all of the aspects of the game world that they need to be familiar with before asking them to illustrate mastery of those concepts. This is true of game mechanics, but I think it's true of aspects of the story, as well.
Gideon the Ninth doesn't begin with the central plot. That would be too overwhelming for the reader; there's too many details of the setting that the reader needs to understand before they could even begin to comprehend what the main plot is actually about. So instead, the first few chapters of GtN are dedicated to teaching the reader those essential aspects.
We start off with a detailed description of the world that Gideon lives in, learn of its magic, learn of the hold it has on her. Then we learn of Gideon herself, her single-minded pursuits of freedom, her strength and abilities in battle. Then we learn of Harrow, which leads to learning more about how necromancy works, and then eventually leads to learning more of the politics of the world. After all that, then the reader knows enough to have an idea of the significance of the Lord Undying requesting that necromancers and their cavaliers travel to the First House to become Lyctors.
Again, in non-speculative fiction, or in speculative fiction that makes use of more familiar tropes, this kind of setup isn't as necessary. A pure fantasy story could've started with "the god-king requests the presence of all princesses and their knights to undergo a test to ascend to divinity" or something like that. For a unique setting such as Gideon's, though, that initial "tutorial" does a lot to help someone a similar level of expectation and familiarity before continuing.
But I don't think this perspective is useful only for introducing new worlds, you know? Every story is written with the intention of exploring certain ideas and themes in more detail than others. I think it could help any story for its author to understand what those themes are, and consequently think through how they would help a reader approach those themes when said reader doesn't initially know what it is they're going to be exploring, what it is they're going to be "tested" on. Starting with the preliminary basics of these themes before moving into more complex stuff can help make sure everyone's on the same page.
It might be a challenge to "teach" the reader about this stuff without the reader even realizing that it's happening, but I think figuring it out is all part of the fun of being an author.