From the title I assumed this was going to be something else. I remember some sites used to port scan you on registration. This was to check if registrations were from an open proxy, which was a very strong bot indicator. I might be misremembering but I think Slashdot used to do it. There were also some plugins for phpBB forums that did it too. I used one back in the day and it helped quite a bit with spam registrations.