Disclaimer: these are only the reasons I go to bars. In no particular order.
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.
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?