My Ph.D research at Clemson University deals with generating perceptually correct soft shadows at real-time rates. The picture on the left shows a scene without shadows. As you can see, it's difficult to determine the spatial relationships between the bunny and the ground plane. When we add shadows (right image), the spatial relationship between the ground plane and bunny become more clear.

   

By adding soft shadows the realism of the scene is greatly increased and the spatial relationships between the ground plane and bunny become even more clear.

 

The soft shadows above are perceptually correct, i.e. the account for the relative distances between occluder, shadow receiver, and light source. Be accounting for the ratio between these distances, the resulting shadows are a great deal more realistic than old soft shadowing techniques (many of which simply uniformly blurred the shadow).