Flat shoulders throw your swing off plane A steeper shoulder turn improves power and accuracy
If you rotate your shoulders on a plane that’s too flat the shaft will appear more horizontal and point way outside the ball at the top of your backswing. This creates a situation where the arms are too wide outside the line, the upper body is pulled back away from the ball and you’ll probably look a bit flat at the top of the swing. This all makes it very hard to swing the club on a neutral path. If you rotate your shoulders on the proper plane the shaft will stay perpendicular to the spine angle you set at address and point at or just outside the ball at the top of your backswing. This creates more tension in the backswing, encourages your upper body to stay centred and enables you to attack the ball on a neutral plane with a more descending attack through impact.