Shape keys are Important for Animation
Shape keys are modifications of the original mesh that you can animate and mix with each other. Previously called Relative Vertex Keys (RVK), one of their major uses is to create facial expressions. The idea behind Shape keys is simple. The basic, underformed mesh you start with is the "Basis" shape.
Can be used to animate lip sync or make poses
Shape keys Good tutorial on why to use a shape key and automating a shape key with a driver. Helps in rigging a character that automatically deforms correctly when you move say an arm or leg.