What does muscle fitness have to do with rehabilitation?

A lot. The stronger the muscle, the more "fit" the muscle, the better one can perform and become stronger and make progress in rehab. It would depend on the overal medical condtion of the patient, the reason leading to rehab, nutrition, and many other factors. If one is in good physical conditon prior to an illness or injury, the muscles will be in better shape to recove and get you back up and going!
A lot. Many debilitating conditions can result in or can be caused by weak muscles. Many rehabilitation programs will seek to recondition weak muscles to allow them to do their job better. Some muscles can be short and tight while others are stretched and weak. A physical therapist can determine this and be of great help.