One thing is clear: Horses aren’t born to be ridden. That we do it is a different story and demands a basic knowledge of the anatomy of the horse.

The weight of the rider is carried on the horse’s back. So that the horse’s spine doesn’t get damaged, the muscles have to be well developed. Young horses need careful training and fully grown ones regular gymnastics.
The old masters knew this: The horse is not here for dressage, dressage is here for the horse.
So you learn exercises on the ground, which will strengthen the horse’s body, stretch and keep it flexible, without inducing weight on its back. The side effect of ground work is that you move and thus gain better coordination, concentration and fitness. Running in the sand isn’t easy, it gives you serious muscle cramps.