Without alcohol sales, HHN would be massively less profitable, to the point where it would likely not be good business for Universal to stage it every year. Therefore, I accept the sale of alcohol as without it, there very well may not be an HHN to enjoy.
As for me, I've never had a drink at HHN, but if I had to guess, a few drinks would probably make the mazes significantly scarier for me. I'm one of those people who can't help but try to anticipate where the actors are going to come from. It's like some weird game I play with Universal, to see if I can outwit their designers and actors, and for some reason I can't just stop. So, I reason that being buzzed would reduce my ability to foresee where actors are going to come from, and make it easier to get scared.