Partitioning becomes necessary if you use multiple operating systems, because each OS must reside in its own partition. And you may have to format those partitions in different ways. And if you install Linux in one of your partitions, you'll have to use its own special file system, known as Ext2. The more you use hard drives, the more fragmented they become, with bits of files scattered across the disk surfaces. All versions of Windows come with basic disk-checking and defragmenting software, which you should set to run at least once a week using Windows' Task Scheduler.
Commercial packages can do the job even faster and more efficiently.