Such as it saves your files, runs faster, saves installed programs, lets you have the OS on it's own, etc. All these are untrue.
But if you insist go buy PARTITION MAGIC and have fun.
I have just got my PC back from repair with XP Pro installed on a 200GB Hard Drive. I want to partition it but don't know if that is possible now it has an OS installed. Do I have sto format again?