Uhm, people may drink because they think they'll look cool, in fact I'll bet there's a lot of social reasons for people's drinking, but I'd go ahead and say that the primary reason to inbibe any mind altering substance is to have your mind altered....you know, get drunk, stoned, whatever it is...seems obvious.
I hate to admit it, because I know that it would have been a bad idea in any other situation, and this'll seem like a whole bunch of excuses, but the only band I was ever in, I drank before and during every show...sometimes a beer or so, sometimes a number of them...It wasn't to keep me from being nervous, I was never nervous so much as excited, but rather because the band was by objective standards terrible and none of us cared about proffessionalism or striving for musical excelence (I did at first, but the undertow of the band kind of took me with it...). Anyway, we were just a bunch of first year college students who had fun playing for all our friends and their friends in the local college town bars and a couple times in other towns...So, we'd get up there, our friends would heckle us and we'd yell back at them from the stage, get drinks thrown at us, things like that...it was a really good time and people liked to see us, but it hat VERY VERY LITTLE to do with the music we were playing at the time... Anyway, I'm not trying to brag, 'cause I'm not good, but I was still the best musician of the lot and always the soberest, so next to my drunk fumbling bandmates I'd look great and get all the compliments afterwards....good times.