I think Jack Bots are random Leveling up allows you to buy custom classes, that is what I have seen so far.
"leveling up" happens automatically as you earn cash. Cash lets you purchase custom classes, pro-tags, and eventually other stuff too, I'm sure. Don't sleep on custom classes though, it wasnt until I was able to customize a character that I really came into my own.
1. Jackbots spawn at set intervals. About every 5 minutes. They spawn on opposite sides. 2. There is no benefit. It's a measure of how much lifetime earnings you have.