You gather a bunch of offenders for weeks or months and then one day they just go poof and everyone knows why.
This way you can weed out the ones who were just experimenting (few attempts) from those who use it regularly.
I thought the point of ban waves was precisely because there's no direct cause-and-effect. E.g., if you perform an exploit and get banned immediately, you know that the system can detect your exploit. If you get banned a month later, it might have been your exploit or something else you did between then and now.
This reduces the selection pressure on black-hats to produce ban-avoiding exploits.