Also it stays optimized and faster longer when defragging software tends to require it again and again to regain the performance. I outlined a safe way here that always gives you a bootable backup to check, plus it really optimizes the hard drive in the process. Perhaps once every few years after many updates, upgrades, program installs and reinstalls may one require it on a Mac, then rarely again. Well that's for Windows because the way it writes files it breaks them all up, OS X doesn't do that on small files, so it eliminates the need for MOST users to require a regular defrag.
You might be experiencing some performance issues with your Mac, spinning beach ball, just general slowness and you knew from your PC days about defragging computers.
