I am not an expert at all on this issue. Newer cars use electronic ignition operated by a computer. So this question only applies to older vehicles that have a distributor and no computer running anything.

What the issue is getting the proper timing from your distributor for firing the spark plugs at the proper time that your engine likes. Gasolene is not as combustible as you might think. In a piston when you introduce a spark to the air fuel mixture, the explosion in the piston is NOT immediate. It actually takes a small amount of time for the spark to ignite the full mixture. The timing of when that spark first starts is important. You want the fuel to ignite when the piston reaches top dead center in the cylinder. If the spark is too soon or too late the engine will not run properly or optimally. Or worse you could have detonation which is a loud knock that could lead to engine failure. So there is an optimum point to do this. The timing set using the distributor affects this.

The engine needs different timing depending on the rpm of the engine. Distributors have mechanical timing advance built into it using springs that are affected by the centrifugal forces, This advances the timing as the engine RPM increases causing the spark to happen sooner.

Where the question came from in the first place was just prior to pollution controls being imposed on cars. In the years before that, GM in particular, put ported vacuum on their cars to cut down on exhaust pollution. Ported vacuum offers no advance at idle and increases the operating temperature which reduces emissions.