It is there to change spark timing when engine is under a load, i.e. driving
In later years the fuel injection computer was used to change spark timing so new distributors were made to use electronic advance instead of vacuum advance
So you either use Vacuum advance, or a computer and electronic advance, you NEED one or the other
If you want to disconnect the vacuum advance to set distributor timing, you can disconnect the other end at the carb, and plug that port while you set base timing.
If you want to leave it disconnected, you may want to consider dialing in a fixed amount of advance and locking the centrifugal advance beforehand.
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.