The Wing Chun practitioner must constantly adapt his speed, and his reactions to the ever-changing situation. He must move his hands and legs in one accord to be able to defend and attack effectively. Doing so requires keen sensitivity. It is through Chi Sau that one's sense of timing is honed to the point of practical application. Skill acquired through Chi Sau will transfer right over to actual combat.
Timing is a principle to be exercised in many aspects of life.