While most of my favorite fantasy novels are human-centric, when roleplaying I'd argue the paradigm shifts somewhat. Fantasy races facilitate roleplaying, and, more importantly, is an aspect of roleplaying many people enjoy. They want to slip out of their skin and into the skin of something fantastical and alien (while still feeling familiar). We can equivocate about the deeper meanings behind the races, but ultimately, most people play DnD because it's fun. They like to play as tieflings, drow, githyanki because it's fun. And that's absolutely valid.
Not to mention aesthetics are a key part of visual storytelling. Spock's design is meant to evoke rigid rationality/intellectualism, and while you can achieve a similar feeling with a purely human design it's not going to be telegraphed as strongly and limits what you can do visually.
And, as already mentioned, fantasy races function as metaphor. Vampires are an obvious example: they can be sexual predators, addicts (sex, alcohol, drugs, take your pick), parasites, outsiders, foreigners, foreigners that bring illness to other lands, marginalized communities, etc., etc. Of course, fantasy metaphors can be done badly--see Bright as an example--but that's an issue rooted in bad writing, not the concept of fantasy metaphors in totality.
“But his mind saw nothing of all this. His mind was engaged in a warfare of the gods. His mind paced outwards over no-man's-land, over the fields of the slain, paced to the rhythm of the blood's red bugles. To be alone and evil! To be a god at bay. What was more absolute?”
