Everything in moderation is key.
Some people just have to put the bottle down because they suck at drinking. Like beeda said, some people just have no control with alcohol. All the demons come out when they drink, and they turn into a totally different person. Sometimes it takes a case, and sometimes just a bottle or two. When I see this happen to someone, I say they're going "to the dark side".
Others, it's for health concerns. The liver needs time to relax, and eventually gets beat up to the a point you get a weak immune system. The 30 days of not drinking is a common thing for a lot of people I know. Although, I haven't tried it just yet, so I'm really going to have to find my substitute. I swear, booze it's the only common thing I have with the rest of the world.