Narrower bars impede the amount of torque you can put on the bars in a sprint. Wider may be uncomfortable. Comfort being important, especially the longer you are on the bike, is where the importance of the shoulder measurement.
If you imagine sprinting, you are pulling up on one drop & pushing down on the other. If the bar is wide you are stretching further, when pushing down. On a narrow bar you are losing some power when pulling on the drop, because your hand is inside the the square lines of your shoulders. It's a leverage thing. As well as comfort, depending on your riding style.
Another example, you are climbing, out of the saddle. Your hands are either too wide or too narrow with the width of your shoulders. In this case you are likely compensating in the same, but different, way.