Lets you monitor how much vacuum the engine has. When you are driving and you stomp on the gas the vacuum will drop almost all the way down to 0, then rise back up when you let off the throttle. For MPG, the goal is to keep your vacuum as high as possible by keeping a light foot on the pedal and accelerating slowly when possible.
Basically, it gives you a tool to monitor your driving style. There may be ways you can keep the vacuum higher, thus saving gas by changing the way you drive. It also can help you gauge engine health.