When executing Soto Uke, what is the critical biomechanical purpose of the circular outward motion, beyond simply deflecting the attack?

Beyond simply deflecting an incoming attack, the circular outward motion of Soto Uke serves the critical biomechanical purpose of redirecting the opponent's force away from the defender's center line while simultaneously setting up a potential counter-attack. The circular motion uses the defender's entire arm and body to change the angle of the attack. If a straight bloc....