It's no secret college kids like to drink. You did it, I currently do it, but it is ridiculous one must be 21 to drink legally. By lowering the drinking age to 18 a few things happen.
The first is the alcohol economy will flourish do to an entirely new demographic being allowed to purchase the product. Second, beer runs will drop dramatically because college students do not have to steal the beer to get it (also helping the alcohol economy). They can now walk up to the counter and purchase it. Lastly, it makes law enforcement around college campus' more focused on real crimes, not just busting underage parties.
Many would counter that it would increase DUI's, but most kids currently take taxi's, walk, or have a DD. By lowering the age, it wouldn't start a movement to drink&drive.