To exploit alcohol deals -- it's sometimes possible to get alcohol at a bar cheaper than retail price (including tip and cover). Presumably it's a loss leader for something else, or the bar is exploiting some sort of bulk discount.

Where are you from? In both Ireland and Italy alcohol is at least as expensive in bars as in supermarkets, usually several times as expensive. (It is a lot cheaper in bars in Czech Republic, but I've never bought alcohol in supermarkets there so I don't know whether it would be even cheaper.) It is not uncommon for people to drink alcohol with roommates before going out.