The light used to illuminate your character and face as if the sun was on your characters face during noon. The shading on it now is very very minimal compared to what it was. It used to stop the weird half shading that occurs constantly in the game where it would make the characters face look strange by giving one solid shade to the entire character and face.

Now when turned on it barely increases the light in the area. (This used to work during daytime and it even still shows shadows during daytime wile activated)