The answer, as it is with many such options, is– it depends.
The app in use was iDefrag, which I’ve used before on Macs in our local office (as much for testing purposes as a functional necessity). Defragmentation, or, defray, for short, is a process which reduces the amount of fragmentation of files on a disk drive.
Back in the day, defragging was very common on Macs and PCs. After a few gazillion rewrites of files on disk, performance would slow down. Defragging often improved performance.
What about Macs and iDefrag? Do Macs need to be defragged? Yes. And no.
iDefrag is rather simple to use. Start it up, select a disk drive connected to your Mac. Select a setting and click the Go button. Then take a long lunch break.
Visually, iDefrag will display files on the Mac’s disk drive as colored blocks, and display the most highly fragmented sections of the disk. When defragging, iDefrag is smart enough to know about OS X’s built-in Hot Zone, compacts free space, and much more.
You can also defrag the Mac’s disk without using a separate startup disk (defrags while your Mac’s disk is running). The first scan will determine how much fragmentation exists and whether or not you need to optimize the disk. There’s a built-in inspector so you can view specific files.
The title question is still valid. Do you need to defrag a Mac’s hard disk drive? OS X has some sophisticated safeguards designed to reduce the need for defragging disks for most Mac users. However, Mac users today have many hundreds of thousands of files on a disk that we didn’t have years ago.
Some of those files are very large (video clips, movies, TV shows, audio files, and large photo files), and if space is at a premium, defragging may improve performance (usually with opening apps, opening files).
If your Mac’s disk drive is a few years old, and you have a few hundred gigabytes of media files, and it seems to run a little slower, defragging may be a good option.