I have mine set to defrag automatically once a month in the wee hours when I'm not home. You gain two things. First, a small increase in reliability. With sequential files, there is a little less chance that the drive will get lost reading the whole file. The second gain is the important one - SPEED. If a file is fragmented, the system has to push the read head all over the disk to read the fragments. Each move takes a definite amount of time. With contiguous files, the head only has to move 1 track each revolution. I have delt with computers that have become dog slow. A defrag session usually brings them back to normal.
