Why do people ask stupid questions?
Okay, so I don't really think your questions is stupid. I'm trying to make a point. What I interpret as stupid is different from your interpretation. What you interpret as "poison" and "pointless", another could interpret as something else.
The simple answer is that people drink because they can. Some feel the need to. A cheap drink to drown out some of your problems is a lot better than going on the streets to get into fights or other methods of relieving life's miseries. Some may not think this is the case, or the right way, simply because they don't have that kind of misery or have learned other ways, but many things are easier said than done.