When data is gathered together for easier access, programs run faster and more efficiently.
Extended Hard Drive Life
When all of the data is in one place, the mechanical parts of the drive have to travel less distance, resulting in less wear and tear on the drive.
More Efficient Security
When your hard drive is less fragmented, it takes less time for an anti-virus program to scan it.
Defragmentation can often reveal faulty sectors that could cause damage to your data if it was saved in this location.