While Mac OS X automatically defrags most of your system, it will only defragment files smaller than 20 MB—a category in which game files rarely fall. Luckily, Steam includes a built-in defragmenter.
Fragmentation of game files can cause quite a strain on performance, so if you're a heavy Steam gamer on the Mac, you may want to check out Steam's built-in defragmenter. Just right-click on any game, hit Properties, and go to the Local Files tab. From there, not only can you backup your files, but you can perform a few cache maintenance operations, including defragmenting the cache. It's not necessarily something you need to do regularly, but since Steam does a lot of large-file copying (especially with games that update often), it may be something you want to check up on. Hit the link for more information.
A Defragging Tip for Mac Steam Gamers [Mac OS X Hints]