Vacuum advance is far more important than many think. If needed to be blocked for timing, simply use a vice grip to pinch-off the rubber hose while setting timing. Vacuum advance is to keep advance in-line with engine load. When vacuum is high timing is advanced, and when it is low, timing is retarded. Climbing a hill load is heavy, so timing retards to keep spark in line with need. As the engine load drops at the top of the hill, vacuum increases and advance increases to aid power. Engines run cooler when timing is accurate, and that is why a dual vacuum chamber was added to later distributors...it increases engine heat at idle to reduce emissions. Vacuum is an excellent way to advance timing with NO computers. For the record I am an ASE mechanic and 50 year custom car builder.