1. Stopping people from creating a second account to slam opposing points of view, drumming up fake support on a subject, or other thread manipulation type stuff.
2. Creating a new user id after their primary user id was suspended/banned.
So, while comparing IPs is definately one of the ways to attempt to identify someone having a dupe account, we always give the users the benefit of the doubt and would at most ask for clarification before doing anything.
By the way, Welcome to the forums!!