This is particularly true in these days of constant security updates as replacing operating system components not only increases the potential for file fragmentation, but also creates a more insidious problem---over time, the operating system itself ends up scattered all over your disk. This, of course, doesn't show up as fragmentation---after all, the files themselves are probably contiguous---but nevertheless, it takes your machine longer to start up, it takes programs longer to start, and above all, it takes longer to get your work done.

iDefrag defragments and optimizes your disk for improved performance. Add in the size of some of the files in common use today, especially music, video, graphics and computer games, and even the best filesystem has trouble keeping things contiguous. Read more.