Shadows provide very important visual cues in the images we see. They clearly show the physical relationship and proximity of different objects. Rendering a 3D scene with shadows can provide almost as much information as a pair of stereo images.

Carlos Castaneda had this to say about shadows in one of his books about Don Juan.

Shadows

Don Juan pointed directly to a boulder in front of us. "Look at the shadow of that boulder," he said. "The shadow is the boulder, and yet it isn't. To observe the boulder in order to know what the boulder is, is doing, but to observe its shadow is not-doing."

"Shadows are like doors, the doors of not-doing. You may say that there is movement in them, or you may say that the lines of the world are shown in them, or you may say that feelings come from them."

-- Carlos Castaneda

A Note on Shadows, Paul Haeberli, Grafica Obscura Oct 1994