To feel the horse's thoughts.

 The rider's hands are not there to control the horse
but to feel the horse's thoughts.
Jean Luc Cornille 2011 

This is especially true with driving horses because 

it is the only physical contact between horse and human.

Why Dressage is Important

 From: Jim Wofford - 2021

James Cunningham "Jimmy" Wofford is an American equestrian, who has competed in many international competitions in the sport of eventing. Today he is most known as a trainer of both horses and riders, and as a retired president of the AHSA and vice-president of the USET.

"Dressage is the most important because it teaches our horses 

to place all their power and athleticism at our disposal."