just like sminlal said. partitioning the drive is useful for organization. it is also useful if you reformat your computer, but do not want to lose your data, for you can just format a single partition.
to partition your drive, i would suggest to resize the partitions before you install the OS. if its windows, you cannot resize a partition within it. though, if you already have an OS installed, but just want to resize a partition, then i would suggest downloading a live cd called Knoppix. it will run on your computer without getting rid of windows, and a program called qtparted will allow you to resize the partition as needed. though, if you do walk that path, just be sure to defragment your drive first