Most Helpful Guy
Unless you have children together, there is no good reason to maintain contact. People rarely remain friends after a breakup. They agree to that because one feels guilty and the other is hoping for reconciliation, but guilt won't sustain a friendship and the other one eventually figures that a reconciliation is never going to happen, so then they go separate ways. Trying to stay friends just prolongs the process, which might be okay if it is so unbearably painful that you need to process it in little bits and pieces rather than all at once.
Most Helpful Girl
Some people do it because they want to continue receiving the benefits of their previous relationship while not contributing anything positive to it.