IP's are used by everything, everytime you click on anything internet related you IP will be recorded, whether it be in an email or just a webpage... all forums keep and track them.
For example if someone tried to hack onto someone elses account deliberatly then the IP of who was trying this would be visable and could be reported, the IP address is held by your internet provider. They can pretty much type it in and have all your address name phone number email details up infront of them.
Theres no possible way to spy on anyone with them... mind you everything on the internet is watched and monitored by someone
All forums on the internet allow administrators to view their members IP address. No IP addresses will be given to anybody else and are kept private from other members.
One of the main reasons for seeing user IPs is to see if people are making multiple accounts however as Kelly said, this can sometimes be a pain if you are using the same forum as somebody else in the house (on the same computer!) If anybody has a situation where they are using the same computer as somebody else when registering, just send me a message so I know that your not making multiple accounts.