Partitioning is not really required nowadas but some people do it to help separate the operating system from thier data in the vent of a crashed op system they can restore / repair the op system partition without affecting the data partition.
I have my master drive partitioned to a 20G partition for my op system (XP) and preinstalled programs.
All programs are installed to a separate partition on the same drive.
All data is save to a separate drive including My Document folder.
Both drives are backed up weekly to a third drive.
Perhaps I'm a little paranoid where my data is concerned but most of my work (lively hood)is stored on the machine.