Shadows can add a lot to a photograph. They can emphasize emotions, create surreal images, and be used in a variety of ways. While strong use of shadows in color photographs tends to be mostly used in silhouettes, it can also appear in a variety of other forms in color photos as well. In my opinion though, shadows are most powerful when used in black and white photographs.
source: http://www.smashingmagazine.com/2009/03/01/showcase-of-beautiful-shadow-photography/