It's funny but I can totally imagine myself doing that when I was younger. If someone had asked me, I would have definitely considered it.
There are tons of reasons--you have something other people need, you're young, it would make you feel good to do it, you get a bit of money from it...
I think the idea of being fecund and generous is very appealing to women.
Lots of men donate sperm--and I don't think it's that different.
You have this incredibly valuable thing to someone else and it is appealing to transfer it to them in some way--The effects on your life are minimal and the effects on their life could be incredibly beneficial. It is a little bit like being personally desired--this aspect of you is desirable. You are helping someone else and you actually benefit to some extent yourself. I think it would be a very attractive option for a lot of people. People like being generous. They like having beneficial effects in the world--and then you actually get some money for doing it, which is rare.
But of course, I don't really know why people do it. I am just guessing. From a few donor profiles I read, I got the feeling the proven donors felt good about themselves--they felt good about the fact they had this 'ability' and they also helped other people.