Shadows are arguably the most simple form of puppetry in the world. I think that strong shadow, or more specifically, silhouette is a key aspect of great puppetry. If a puppet’s pose or movement reads well in silhouette it clearly communicates to the audience visually what a character is doing, thinking and/or feeling.
“Silhouette value” is a fundamental concept in many art forms. John K. has written about the role of silhouette in animation a lot and Spline Doctors touched on the subject recently as well.