partitioning splits the hard drive to emulate multiple devices. One of the benefits would be that you can store all your files on one partition, and your OS on another- then if you ever need to reinstall your os, you dont need to format the entire drive and thus worry about loosing/backing up all your files. You just format the partition with the OS, reinstall it and you up and running in no time.
I use a partitioned drive to install a few operating systems- windows + ubuntu/kubuntu etc I can then play games in windows and do all my work in the far more secure linux environment. Not only that, but in linux programs can only access other partitions after a password has been keyed in, so if some malicious script does happen to run, its not going to be able to get to my system files anyway.