For the record, though alcohol is banned for under 21s in America, my brother said they just smuggle it in the dorms etc. It's just more undercover.
I think it's partly a sort of social expectation - a lot of people go out and expect to get shitfaced. I don't think that they purposefully go out with the expectation of getting really sick and throwing up everywhere, but sometimes things don't go quite as planned and it ends up happening (especially mixing drinks).
Social drinking
Drinking a bit for confidence when trying to pull
Drinking because you always drink and getting really drunk
