what country club schools did you people go to?.. with gym clothes provided and such. It seems weird if you ask me; I'd much rather wear my own clothing.
BYU. They had several reasons they did things that way.
1. They used it as a type of rec pass. If you were in official clothes, they knew that you had used your ID to check them out from the locker rooms (or had obtained them illegitimately). You were still supposed to have your ID, but as long as you were in clothes, they only checked for intramurals.
2. It helped keep the locker rooms smelling a bit better. You didn't have folks storing the dirty clothes in the lockers, and taking them home once a semester whether or not they needed washed.
3. It was an easier way to enforce the dress code than having to tell folks that wife-beater shirts and bum hugger shorts weren't allowed.