Let's say you have private or paid forums that you have only granted access to certain groups.
If you suspect a member has been passing around his log-in credentials, this would be a way to check up.
If a user logs in from one ISP (let's say AOL) one minute, then a minute later from another one (say Comcast) it's a safe bet it's not the same guy.
Of course, there's a chance it could be the same guy, but if it happens often enough, and from differing ISPs, you know it's not the same guy.
I don't even think this activity is going on on my site, but I'm just trying to plan ahead.
