You make an account called "BLOKLED SUKS ASS" because you're the pr0 trollr and you dont enter a key. After a month of forgetting about that user it is automatically deleted. You go "oh ill try again" so you make another account with the same name, still not registering a key but every three weeks you do a simple log in. You do this for a few more months because you're insane and want this keyless user to stay alive. Then you forget about it. After a month, it's deleted again. FFFFFFFFFFF.
Another more friendlier example is that you're interested in the game but don't have a key. You make an account anyway and log in every day while you browse the forums. Your account isn't deleted because you keep logging in. However, another person may have done the same as you but then decided "meh im not interested anymore" and simply stops using his account. Because he hasn't logged in for a month and he has no key registered, his account gets deleted.
This could be used to clean up spam users/bot users etc.