If your hard disk is over about 90% full, defragmentation may help as well.If you've downloaded many large files (such as video files) over time, your Mac may benefit from defragmentation.Files that have more than eight fragments.Files that are less than 20 MB in size.These are some additional conditions in which Mac OS X will automatically defragment files and reasons that your computer could benefit from defragmentation:
However files can still become fragmented depending on how many times they are opened and if there is not a large enough amount of space to fit the complete file. OS X (10.2 and on) automatically decreases the potential for file fragmentation by writing the new data to the largest block of free space on the hard drive.
