At part throttle, the air/fuel mixture in the combustion chamber is very sparse, so it burns much slower than the piston travels away from it. For efficient use of the fuel to make power, the maximum flame spread and cylinder pressure needs to occur near the beginning of the downward movement of the piston on the power stroke when it has the most mechanical advantage and before the exhaust valve starts to open. With the slow burning part throttle charge, it needs a bigger head start to be maxed out in the sweet spot. Vacuum advance gives it the head start it needs, so the piston is not past 90 degrees atdc when the flame front is at its peak. Ever try to pedal hard on a bike when the pedal is past 90 degrees? And the exhaust valve on a performance mill will start opening +/- 70 degrees before bottom dead center. Lack of advance will send part of that still expanding charge out the exhaust.