The goal itself is motivating. It gets people to give that extra effort and to keep on pushing. Vague goals are worthless. But a quantifiable goal isn't enough. A valid stretch goal also needs to have measures that identify progress toward it.
Workers are much less likely to slack off when forward motion is continually monitored.
But Apollo had feedback mechanisms that allowed it to make rapid course corrections. Stretch your workers, in other words, but don't break them. Inc. All rights reserved.