In general, lots of reasons: love someone and want to be together a LOT, want children in wedlock, don't like being alone, financial or emotional support...sometimes just as an escape (my mother "ran away from home" that way, at FIFTEEN!!!).
My first marriage was kind of a convenience thing (although emotions were involved too) - we were both military and that was a way we could be (for the most part) stationed together. The things we had in common as a military couple didn't last as a civilian one. But my son came from that union, so I have minimal regrets.
In a sense, my 2nd marriage was a bit of a convenience as well. I loved DH (still do...lol), but really didn't want to get married. I'd been single/independent for over ten years, had my own home that I'd scrimped and saved for, a son in school that I didn't want to uproot, etc. I was very resistant to changing any of that. But driving back and forth (about an hour drive) several times a week got old (for both of us) after a while. And the schools in his area were excellent. So I took a leap of faith in remarrying, and other than the rough first year of transition, haven't regretted it.
I know we'd all like to believe (because it feels so nice) that most couples get married for love, but there's really a LOT more to it than that - even throughout history.