Consider the observations of Pennsylvania physician Dr. Louis Wagner:
"A horse’s walking action mimics your body action. So when you put somebody on a horse, the horse moves and the rider’s muscles move in synchronization with it. Unlike exercise machines, a horse makes your whole body respond in a smoothly rhythmic, progressive way."
This is why many physical therapists join their patients for therapeutic riding: they can do things (at the riding center) which can't be accomplished in the therapy room.
The riders benefit physically and emotionally, gain strength and flexibility, learn consequences, and feel a sense of accomplishment during the riding sessions. These benefits carry over into their daily lives.