There are different kinds of anonymity. If you just want to hide you IP address from a specific website's operator, then you don't care much about being trackable or not. You just use Tor as a fancy IP spoofing technique. They know that you are you, but might find it harder to ban you or call the police on you. (Of course, they can just ban all of Tor. And of course this scenario is mostly useful for trolls... but I can imagine some cases where this you have a legitimate need to spoof you IP like this.)