Buzzsaw On-The-Fly Defragmenter allows you to defragment your drive while you work.

Buzzsaw had on-the-fly defragmentation before any defragmentation program on the market made it available. This program runs at idle priority so you can continue working without Buzzsaw using your valuable CPU time. It watches the drive for modified files and waits approximately 60 seconds before defragmentation begins.

It waits, because you may be downloading a file, etc. and there is no need to defragment the file if it is going to be modified again shortly. If you are a developer, you may be compiling files, or saving files frequently which is another reason for delayed defragmentation.