Silhouettes remove all the distractions and clutter in the scene and distill the image down to its most basic, this is why the story is so important. Just using shapes you must make a photo that is recognizable, and that the viewer can relate to. While this sounds easy in practice, it can be difficult to achieve. You must have the right elements. The light needs to be in the right location, the subject needs to be facing the right way, and they need to be in a pose that makes their action recognizable.
Learn more in this article by Craig Colvin at the Digital Photography School.