What is the benefit of partitioning your drive?

Partitioning allows you to create a logical separation between different operating systems and data.

For example, you could have a Windows operating system on one partition, a Linux operating system on another partition, and your personal data (My Documents in Windows and /home in Linux) on a third. This allows you to wipe one of your partitions clean and reformat without having to backup the data on the others (say for example, your Windows install gets a virus).