A hard drive can be crammed full of stuff you don't need, or want. Windows applications litter your hard drive with junk that serves no purpose. This junk can cause your PC to slow down and even cause some application problems. Some programs use temporary files to hold intermediate data but do not delete them after program shutdown (e.g. just take a look in your Windows\Temp directory where many such files end up). Many setup programs are prime examples of this. Other programs keep producing lots of unwanted log files. Yet others automatically create backup files of edited documents. Internet browsers create a lot of cache files, 'cookie' files, history files, etc, etc.

The problem is that if you don't clean up such files periodically, your free disk space will slowly but steadily shrink and it will be more difficult to find the files that you do want because of all the junk littering it. If you are really unlucky, Windows may even crash because it hasn't got enough space for its virtual memory swap file. Read more.