Playing races for physiologic reasons.

[Polycarp]

I had a player who's character was actually an intelligent +3 legendary dancing longsword. If that isn't a physiological difference than I don't know what is.

Well, it's not so much having a physiological difference as "having no physiology."

[Xeviat]

The 4E PHB had these (annoying) little "play this race if you want to ..." entries. For dragonborn, one of the reasons to play them was: if you want to be a dragon.

Otherwise, I have a flying race and a swimming race in my setting, and I think a lot of players will play them for those reasons alone.

It's also possible that people play dwarves, gnomes, and halflings because they're small. And, though this one's a bit more psychological it is due to physiology, Elves traditionally live a long time, and I think it would be interesting to play a 600 year old elf (in settings where they live 1,000 years).