1. A partiton is a piece of your harddrive separated from another part of your harddrive, and your operating system will see each partition as a seperate harddrive.
2. If you have more than one OS, or want to keep important files seperate from the main harddrive.
3. Because you don't need it...?
4. Using Partiton Magic, fdisk, cfdisk, QTParted, GParted, or any number of partitioning tools out there.