Use Solidify deformers when you want to keep deforming areas of your geometry solid (or non-deforming). This can be useful when you want to create solid spikes on a deforming dragon tail, or for rigid pieces of equipment, such as a belt buckle, modeled as part of the deforming clothes of a character.
The Solidify deformer analyzes the deformed section and divides it into separate polygon islands to create the look of sections of the geometry becoming more solid and less stretchy.
This effect is achieved by replacing each polygon island by the Original Geometry moved and blended into place.