The vacuum advance gives additional ignition advance at part throttle cruise conditions. Can make a big improvement on your gas mileage, and doesn't harm your performance because it isn't operational at wide open throttle. A dual vacuum was an emisions gimmick Ford came up with in the70s that will actually retard ignition timing. The retard canister is the one closest to the distributor body. Just about everyone used to remove that vacuum line and plug it.