You asked what you gain by this, I can tell how it works.
When you save something to a disk it is a file which consists of many clusters. These are saved beautiffully by each other. The problem starts when you erase something. It leaves a clean space like the clock on the wall when removed, and your winchester will fill that hole on thenext saving. If the hole is smaller than your newly saved file, it will fil the hole and searches for the next free space on the fragment table. Like this the file will be saved on your disk in three-four-five or more pieces. Also your files can damage. You know when the head reads the information it spins like crazy, for it has to collect information from different parts of the disk (you see the fragment table full of holes and as colorful as a toy pony). You help your winch by defraging to find your files easier, quicker so the lifetime of the hard disk will increase.
