Dressage is the FOUNDATION of every form of equestrian sport.
Dressage basically means gymnastisizing your horse and the rider schooling the horse to be obedient.
These aspects make it the foundation for every form of equestrian sport.
The higher the level of dressage you achieve, the better the communication between you and your horse will be. Your horse is sensitive to your aids and understands what is expected of him or her. To understand dressage you have to understand the horse itself.