A few reasons, sometimes in combination, sometimes not.
Perspective. I like to explore an inherently different perspective from my own. In a world that contains both humans and Goliaths, I can’t sleeve into the perspective of the biggest, strongest, stoic danger-bear in the room. Or the little guy who fools don’t take seriously because of his size and friendly demeanor.
Beyond that, a human simply cannot have certain perspectives, like leaving home as an expert with 80 years of study in a subject, who isn’t fully considered an adult, socially. Or being a person who grew up having conversations with small animals. Or whose whole family and society is magical.
The Look. I get an image of a small person riding a war dog in half-plate, talking to his falcon, and I want to play a Forest Gnome Paladin or Cavalier.
In addition to those, sometimes the place in the world of a given race simply appeals to me or allows me to explore something that I wouldn’t be as close to as a human.
For instance, my gnome rogue/wizard is a swordmaster trained in an art from before the fall of the great northern kingdom of winter fey.
He also lives on the mountain where they ruled from, and his family line traces back to that kingdom and some of the heroes from the war that ended it.
His backstory involves a lich who came to the mountain for power, and a friendship with a clan of Goliaths in the high mountain.
Because my character is a gnome, his home town is higher in the mountain than the humans live in this region, his blood is tied to the ancient kingdom (a humans wouldn’t be, they’d be a relative newcomer to the region), he has a deeper cultural knowledge about the Demon War as it relates to the north, his Blade Song is tied to that ancient sword art (basically the Magical/Spanish/Thibault’s Circle with actual magic) that his uncle taught him, etc.
He’s part of the setting, his gnomish history is tied into his personal story and the campaign’s main villain, and I get to explore this whole culture and history. While some of that would be possible with a human, in order to have that history element he’d have to be from a very different place with a very different culture.
Finally, sometimes a race has themes that intersect with something I’d like to explore with the distance allowed by roleplaying a character who is kinda like me, but also very not.