More hard drives equals more storage! Also, if you are partitioning one physical drive into other partitions than you can manage files a little easier. Also, Windows treats these as seperate drives even though they are all on 1 physical drive. So, if you had all of your system files on the c partition and maybe some music on the d partition, when you are locating some music on the d partition, the drive needle only attempts to access files from the physical area that the d partition is located on.....faster searches!
Another tip is to place the pagefile (system file that windows uses on the drive to mimic RAM) on it's own partition. So this way, the pagefile never gets fragmented and windows can access this file faster. Does this make sense?
Another common and effective reason......organization!
-Mike