I unfortunately only have four snakes right now(3 balls, 1 boa) and a large tegu and I breed rats.
1: To, of course, feed my reptiles. 2: I actually enjoy breeding the rats.
It's fun to see the different colors and patterns you can get. And because I live in a very populated area in southern California, it's not too hard to sell off any extras and make the money back to cover a lot of the food and bedding costs.
The only negative thing I have to say about them is that I hate males. In my opinion, the males smell terrible very quickly. I do have to clean their tub more often than any other.