People typically like either use light text on a dark background or dark text or a light background. With the invert feature in OS X, the user can quickly jump from one to another. This is frequently nice when trying to use your MBP in direct sunlight.
Come on, you are probably using a mac right now, so just try it.
Hit it again to reverse the inverse.
Why is this useful?
The main time I use it is when I am coding outside. A dark text on light background gets washed away in the sunlight. By doing the invert trick, it changes to a light text on a dark background which is much easier to see.
If one of your mac-owning friends doesn’t know this trick, it’s an excellent gag as well. “What the heck happened to my screen!?!?!” 🙂