We can't pay for our own sins. Remember when you busted the neighbors window when you were a kid and you had to mow the lawn for a couple of weeks? The work you were doing couldn't possibly be worth the price of a window, but you still had to do something to pay for your deed.
Purgatory is the same thing. We can't possibly hold the wiegh tof our sins, so Jesus did for us. But you still don't get a free ride.