I do not know. But they had that option in certain games I had played. Specially the FPS ones. Some one must be using it, that's why they have the option.
And games like Witcher 2 had no option to Invert Mouse at all. It's such an awesome game but I spend lots of time to get used to normal mouse setup. And after I finished the game, I did not know if I should keep it inverted or normal.