Demon Lords are, of course, only one of the many varieties of archfiends, as demons are only one variety of fiend in the D&D game. The Archdevils, the lords of the daemons, and more. Of course, the variety and diversity of fiends in D&D is somewhat bizarre and hinges heavily on the philosophy of nine-point alignment being a driving force in the campaign settings that are built around the game. Although sure, there are a few minor statblock differences to set them apart, including what energy immunities they have and what their alignment says, how does one really distinguish between a devil and a demon? Or between either and a daemon? Or demodand? Or rakshasas or hags? What about critters like the efreet and the slaad? Not actually fiends, but conceptually (and in every other way) how do you tell the difference, really?
Looking at fantasy settings that are not based on D&D exactly, but are clearly very, very similar to it (such as the Warhammer world) we get Daemons of Chaos--which, again, are conceptually very difficult to tell apart from fiends in D&D. We even get the Bloodthirster, who is almost identical to the balor (who in turn is almost identical to the balrog of Tolkien; the obvious immediate point of inspiration for both.) In Cthulhu Mythos terms, we get the entities of the mythos frequently described as demons, and behaving in many ways exactly as such--yet in D&D, they are from the Far Realm and--again--are therefore, not fiends. And if you go in for the Pathfinder setting, you've got to add divs, asuras, qlippoth, oni, kytons, and more.
More and more, I see fiends in D&D becoming insular, Byzantine and esoteric. What is the purpose of all these divisions? And if we have all of this, why is it that the demons seem to get the most use?
Personally, I don't think that there is a valid purpose anymore to the esoterical divisions. I think a lot of that is legacy stuff that has lingered due to tradition and inertia, frankly. And I think demons have the most popularity because they are, in some ways, the most straight-forward to use. Their more open heirarchy, with rival lords ruling over a balkanized Abyss, also makes it easy to slip in all kinds of things without feeling like there is a potential domino effect on established canon--something that's harder to do with the more heirarchical devils, for instance.
I'm a huge fan of these monstrous, Machiavellian outsider lords. Between D&D and Warhammer, I've long found the notion of named, powerful, fiend lords to be a compelling and intriguing concept. Not only that, much of the "lore" we have for these fiends comes from not just pagan mythology, but esoteric Judeo-Christian traditions. Many of the archdevils and demon lords are not unique to D&D (or Warhammer) but are in fact demons and devils from Biblical or extra-Biblical sources, or from the pagans that surrounded the Bronze and early Iron Age Israelites, and thus were temptations for them to avoid. Archfiends such as Beelzebub, Asmodeus, Demogorgon, Pazuzu, and others fit this bill. This rich legacy, coming as it does from our own cultural roots, makes them even more iconic as villains and antagonists to be feared in the game. Given the problems I noted above, though, when migrating the notion of fiends and their lords into DARK•HERITAGE, needless to say, I did away with some of the esoteric and arbitrary divisions between the groups, and adopted a more balkanized, "open" format for their various courts and kingdoms that more resembles the Abyss from D&D than it does any other lower plane. Fiends can freely mix and mingle in any given court, driven more by what makes sense given the proclivities of the court's lord and the environment of his particular realm than by any division or label created by the game.