Turning your left shoulder downward instead of level helps to keep the center of your shoulders in place, which is a key move of Stack & Tilt. With the shoulder center fixed, the precision with which the club hits the ground increases. Aside from promoting good contact, a stable axis for the shoulder turn helps keep the path of the hands and arms traveling on a circular arc around the body.
Imagine a spike running through the top of your spine and planted in the ground in front of you so all you could do is turn around a fixed axis. Stabilizing the shoulder center like this means the swing will tend to bottom out wherever you set your weight at address. If the weight is forward, as we prescribe, the club will hit the ground after striking the ball.