Fast attack units wait at the edge of a battle until the grunts engage the enemy, then they sweep in and attack the enemy from the side or from behind. This is a devastating tactic. An army cannot fight well when it is being attacked from two directions, and a Fast Attack group will inflict much more damage than it recieves. Sun Tzu said, "Use your normal forces to engage, and your auxiliary forces to win." This gives you an idea of their importance.