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.