To drag things back from alignment
What benefit does it have? Are there too few fey creatures?
Generally D&D's been pretty low with actually hostile Fey creatures. The closest have been Fomorian which, well, haven't been treated as Fey, they've always been Giants, and Hags haven't been Fey for the longest time either.
More things under the Fey label does give more room for building thematic encounters. You need minions for Balor the Fomorian king? Look at the nasty Fey stuff and pick some out appropriately. Previously stuff mixed in fiends for their dark fey or were just lazy with the "Okay you can use an evil dryad, an evil satyr and.... A redcap and a quickling (they're both evil brownies, except the quickling is fast)", meanwhile goblins got left in the trashbin of "We need a low level mook and they are eternally this" despite, y'know, pop culture stuff and Labyrinth
So are dragons, monstrosities, aberrations, undead, giants, elementals, fiends, constructs, plants and beasts.
Fey get a whole Shakespheran play about them. We really arguing Fey don't have massive chops in western mythology?
Dragons are not the core of western mythology, though they are a primarily source for French town myths, but once you get outside of the Tarrasque and St John they're kind of non-existent
Monstrosities have no unifiying features, they are not the core of western mythology
Aberrations sure aren't the core of western mythology, they weren't invented for years later
Undead have some showings but they're associated more as threats and not grand mythologising
Giants I'll give you a few appearances in western mythology, but they're generally limited appearances outside of being one of the Paladins and the eternal enemies of Don Quixote, but how many appearances in Shakesphere's works do they get compared to the fey?
Elementals, you might get away with saying Ariel is one but can you exclude him from being Fey all the same?
Fiends is another one I'd put with undead in terms of being seen as a threat and not something being told stories about
Constructs are not a core of western mythology. They got a fair few more showings than a few of these others, with the Golem, Talos, Galatea and Frakenstein's Monster, but a core?
Plant monsters are not the core of western mythology, as fun a film Little Shop is
Beasts are not the core of western mythology and fit into that threat side of things
Fey are the ones who got fanciful stories told about them, rumours of their existence, and always existed at that periphery. Fey are the ones D&D's ignored for years despite what an easy wellspring it is to draw from, and how popular "Woops its the Fey but they're spooky now" is as a genre.