As you delete things from your computer, you end up leaving a "hole" in the disc. You computer puts info where ever it can so it fills up all of those holes with more files. You may have what you think of as one file and it may be spread all over your hard drive in little pieces. It really slows your computer to access these little pieces. The defrag puts all of the pieces in one place and thus speeds up your machine. It can also eliminate a lot of lock up problems on older machines.