As files get added and deleted, they start to leave gaps in HD storage and other files get written into those gaps. Steadily, files become more and more broken up.
By defragging, you can make your files continuous again, increasing read speeds. I've seen benchmarks which call the effectiveness of defragging into question, but its generally seen as a good habit for system maintenance.
So nice to see a non-HL2 thread here once in a while.