I mean, aside from keeping it for a purpose of banning, multiple registration from same PC, and counting online users (not sure about counting users though). The, what is the need for keeping an IP of the user? I just need to know more, because if it is beneficial I need to use it.
Its also used to find the location / origin of the IP , so that if there is illegal activities going on using that IP , Law enforcement authorities can use it to track down the users
And the location is useful for other things, like setting the default language, the timezone or, on the right kind of site, locating the 3 nearest stores to the user (or some other function like that).