Microsoft's housekeeping utility encourages you to regard your hard disk as a well-ordered filing cabinet, full of neat little folders and tidy files. It isn't neat. In fact the average hard disk isn't really hard. It's flabby with digital scraps whose greatest desire is to become a form of data soup. Clusters want to get lost.
Files yearn to be fragmented.
Your only defence against hard disk disharmony is a powerful file management utility which, Windows Explorer isn't. It's easy to use, and for basic things like moving and copying files, it's adequate. But if you want to be able quickly to find your files, individually or in bulk, search them, copy, move and rename them, change their attributes, archive them and see immediately how much space they're taking up etc. Back in the dim mists of time, when Bleeding Edge first started using PCs, we used to use a program called XTree. XTree Gold - before it was released in a Windows version called XTree Win.