You need Vacuum advance or engine will stumble when accelerating or be very sluggish
Gasoline engines have two types of vacuum, regular/direct or ported
Direct vacuum is in the intake, between intake valves and throttle plate, carb or EFI, high vacuum when throttle is closed
Ported vacuum is between air filter and throttle plate, high vacuum when throttle is open
On a Carb direct vacuum is below thottle plate and Ported Vacuum is above throttle plate
You always use direct/regular vacuum for Vacuum advance on a distributor
When emission requirements got stricter most car makers used Ported Vacuum to cause exhaust leaving a cylinder to still be burning, this is before catalytic converters, when car makers were using Air Injection to lower emissions
This caused all sorts of exhaust valve issues, lol
And some used thermal switches to use ported when engine was cold and then changed to direct after warm up, or visa versa
The point of Vacuum advance, or LOAD Advance which is what it is, is because air/fuel mixes burn at different rates
Leaner mixes burn slower(however leaner mixes can self ignite easier, which is something different)
Richer mixes burn faster
Distributor(or spark module) can do RPM advance on its own, with weights and springs(or timing chips), and thats pretty simple, and this is what "Base spark timing" is being used for, RPM Advance
The point of spark timing is to ignite the air/fuel mix Before TDC, so you get full explosive power After TDC, 5deg to 10deg ATDC, so piston/head chamber is still small and piston/rod has leverage to push down in its crank journal
The base gasoline 14.7:1 air/fuel ratio has a known burn rate
But as you richen the mix burn rate speeds up, and its not "fixed rate" the richer it is the faster it burns, until it floods out the spark, then no ignition, lol
So when you "step on the gas" you change to a richer mix which burns much faster, this means you need to change the RPM advance spark timing, reduce it
Say RPM advance is set at 10deg BTDC, base spark timing
When you hook up Vacuum advance it should go up to about 20deg BTDC, because direct Vacuum is high at idle, so the higher vacuum is holding spark advance higher
If you slowly raise RPMs, spark timing will slowly go up, RPM Advance only
If you "step on the gas" vacuum drops and spark timing will as well because the Richer Mix will burn/fully ignite faster so needs spark to happen later, closer to TDC, to get full ignition at the 5-10deg ATDC