This is useful when you are multi-tasking on the Mac and you get a bit overwhelmed and lose your way in all the chaos. The three-finger gesture also allows you to start a new desktop session. This way, you could have a web browsing session on one desktop, and run some graphic design apps on the other desktop if you need it.
This brings a good sense of order to your workflow and makes things infinitely simple to understand what is actually going on.
