I am curious as to why you think it is useless. If you are getting no problems on your pc after numerous installs and uninstalls then patently the defrag operation is doing its job.When a file is too large to store in a single location on a hard disk, it is stored on the disk in discontiguous (not adjacent) parts or fragments. This fragmentation is "invisible" to the user; however. The locations of the fragments are kept track of by the system. Over time, disk access time can be slowed by fragmentation since each fragmented file is likely to require multiple drive head repositionings and accesses. (There's nothing you can do to prevent fragmentation, by the way.)
A disk defragmenter is a utility that rearranges your fragmented files and the free space on your computer so that files are stored in contiguous units and free space is consolidated in one contiguous block. This also improves access time to files that are now contiguous.