It is an interesting feature of human beings, revealed I think by role playing games, that while we like thinking of ourselves as individuals, we also quite like the idea of being exemplars of one of a smallish number of archetypes.
The obvious example of this is star signs, as I've written about before. For some reason, while we all imagine ourselves as special snowflakes, at the same time we are also frequently willing to accept that we are members of 12 distinct divisions within the human race that to some extent define our personality in the same way as for others within our caste.
Similarly, part of Harry Potter's ongoing appeal seems to be the idea that young (nowadays not so young) people can feel affinity with one of four houses, which seem to indicate that one is variously good and brave, clever and swotty,
evil 'resourceful and cunning', or, er, a bit shit. The same is - I can attest - true in English schools in general; in my own school there were three houses who for some reason were considered to have certain characters even though this patently couldn't have been the case in reality. (One was 'swotty', one 'sporty', and one - inevitably - 'remedial'.)
But RPGs are particularly ripe with this kind of thing. D&D is the obvious illustration: one is not a unique adventurer in D&D, but an elf or a fighter or a cleric. The idea that the appeal of this is really just a matter of division of labour is not I think really true; it's much more to do with the individual liking the idea of adopting a persona that is to a certain degree archetypal. This is why most D&D players still like to be able to make these kinds of choices even when there is no real mechanical benefit attached - it is important to be able to say, "This time, I am going to be a dragonborn and it is going to affect my PC's personality accordingly."
The apotheosis of this was probably the mid-late 90s, when the most popular RPGs all seemed to come with big lists of archetypes which one could adopt. (Cyberpunk 2020 and Shadowrun are the ones that come to mind, but think in particular of the oWoD games, which not only each came with a dozen or so 'races' that one's PC could be a member of, but also cross-cutting divisions like Changeling's seelie/unseelie and young/wilder/grump, or Werewolf's different phases of the moon - so that there were archetypes, sub-archetypes, and sub-sub-archetypes for you to choose from.) There was a reaction against this in both the OSR and amongst storygamers, but it is clear from looking at the 5th edition materials that things haven't changed all that much in the period since.
Why is this? Clearly, part of it is to do with one of the well known, but not well remarked-upon aspects of role playing games, which is that they provide a way for people to imagine how they would conduct themselves if they belonged in a different skin - as an extension of the childhood impulse to imagine oneself in archetypal forms ('I'm a cowboy'; 'I'm a cat'; 'I'm the planet zog'). It's easier to do this if one can, symbolically, think of oneself as stepping into a kind of pre-made costume or suite of characteristics than it is with coming up with something entirely new on the fly.
But I think it's also because we tend naturally to categorise ourselves and each other and indeed the world around us, and this indeed seems to be a feature of how we interact with the world: there are different animal species, different species of trees and plants, and also different categories of human (and here I don't mean in the sense of different races or ethnicities, but in the sense of different personality types: the nerd, the jock, the cool kid, the sneaky politician, the femme fatale, the dirty old man, etc.). We are comfortable with this, and it seems to strike us as natural. We are individuals, but we also fit into conceptual groups or tribes that do not map to race or language or place of origin.
In any event, this way of approaching setting design seems to be popular because it taps into something deep in our psyches. Anyone who wants to design a role playing game or write popular YA fiction take note: if you want it to sell, make it tribal.