Why is public drunkenness a crime?

Public drunkenness, a.k.a. public intoxication, is something you can be arrest for or charged with.

Why???

Selling alcohol in public places is legal, subject to certain licencing requirements. Drinking alcohol in appropriately-licenced public places is legal. Driving under the influence of alcohol is very much illegal. Drinking alone is socially frowned upon.

So basically, if you want get a perfectly legal and perfectly socially-acceptable buzz on with people who don't live in your household, you're going to have to be drunk in public at some point, even if it's just on your way home.

So why is this a problem?

If they're worried about disorderly conduct, they can charge people with specific disorderly conduct. Assault, robbery, harassment, disturbing the peace, whatever. But why make my tipsy walk home from the subway illegal?

Some might argue that it's inadvisable, and it may in fact be inadvisable depending on inebriation level and environment. But that's no reason to make it illegal!