Why was dressage invented?

Dressage was originally used to train cavalry horses. The horses needed to be very attentive to the rider's aids, as well as being able to pirouette to enable the rider to fend off an attacker. Originally, high-school movements, now seen in the Spanish Riding School and Le Cadre Noir, were training as well. The leaps and 'goat kicks' were very useful for the military equestrian.