Mesh DeformerBasic Coord. Object
This Deformer, also known as Cage Deformer in most applications, allows you to create a custom low-resolution cage around your objects that will be used to deform them freely. It behaves a bit like the FFD Deformer, with the added flexibility of creating your own cages using any polygonal object.
So what could it be useful for? Many things.
For example, it could replace a lengthy joint/weight painting setup if you only need to do some quick deformations. It can also be used in conjunction with Joints, of course, since this low-resolution mesh can be used to drive a much higher-resolution one, which would be problematic to weight properly to Joints. Instead, you can weight your Joints directly to that low-resolution cage, which will in turn deform the high-resolution mesh.
Since the cage is polygonal, any other Deformer or Expressions can be used with it, such as Pose Morph.
The Mesh Deformer can also be used to add deformations on top of Joints by simply weighting the high-resolution mesh along with the low-resolution cage, so that the cage follows the movements of the mesh, allowing you to add deformation whenever necessary. That way it could, for example, be used to create complex squash and stretch effects.
This tool can also be used for modeling to quickly change basic proportions in a character in a non-destructive way. This is very useful if you are unsure of the direction you want to take, or if you have an indecisive client.
The possibilities are vast, don't hesitate to explore!
Note that the Mesh Deformer does not work in conjunction with Primitives.