At first I though it was just a little fun thing they added to Mac OSX, but today I discovered that it actually feeds a purpose. I was reading an interview on a dark website and half way through my eyes started to hurt. So I used the shortcut to reverse my screen colours which allowed me to read black text on a white background which really helped getting through it. Below are screenshots of what it looks like. I used this site as an example.
To access this shortcut press: ctrl+alt+apple+8
Hit it again to reverse to the original display.