Mr. Casey (the dog) has multiple sclerosis in his hindquarters so his back legs just wouldn't work - he couldn't get about at all before he was given wheels. 

 

The little chariot contraption with 2 wheels lets Casey go to the park, and cruise along the sidewalk. 

 

I've seen a German Shepard who used a similar rig - but the Shepard has his back legs on top of a little platform.   Casey's rear legs have little rubber boots - so they can still try to provide a little mobility, but won't get scuffed if they are dragging along.

 

A bit of a sad site to see, but I can see why one would do it.