Great question, and an important one.

I think people get married because:
- Society expects it. It is seen as a measure of a person's success in life.
- To have children: Some folks do not believe in children outside of wedlock (I'm one of those people).
- It is for two people to show the world, and themselves, the level of commitment they have towards each other. Why not just stay bf and gf, or even domestic partners ? Because I wanna show you how much I love you.
- Money/Celebrity. I live in a place where this happens all too often. Disgusting to say the least.
- Stability/Predictability: This is HUGE, I think. It really helps to be able to count on something being true everyday when you come home from work. There's the smell of hot soup and pasta coming from the kitchen, and there's someone you can tell about your day.

Given the success rate of marriages, I, personally, am not sure I'd ever get married. I don't think there's anything wrong with just being bf and gf forever. And, divorces are unnecessarily NASTY.