Layers allow you to add layers of animation above the original biped animation. This is a powerful way of making global changes to your character animation. For example, simply add a layer and rotate the spine forward at any frame, and a run cycle becomes a crouched run. The original biped motion is kept intact and can be viewed by switching back to the original layer. Layers can be viewed individually or as a composite of all the animation in all the layers. Layers behave like a freeform animation; the biped can adopt any position.
Layers will allow you to easily adjust raw motion capture data, which contains keys at every frame. Simply add a layer and keyframe the biped.