You wouldn't believe the number of forums I had to register at just so that my little cousin could show off his cool new sig, the cool new animation he made for a forum buddy of his, etc. So I just sign up, check it out, and never step foot in the place again.
But if he wants me to see some other stuff in the same place once again, it helps to always use the same user name and password wherever I sign up, just so I'd remember.