Adjustable Vacuum Advance

I agree that vacuum advance via plugging the can into manifold vacuum can be a good thing.
At low engine RPM, like at idle, the air-fuel mixture can have poor mixing due to a lack of turbulence. Certainly the AF mixture is not very dense. To get the mixture in the cylinder to burn its best it can be helpful to light the fire early.
That will keep the spark plugs clean and help off idle acceleration.
The major OEM’s spend countless hours and dollars figuring out what is best for the engines that they produce. On an engine that has been modified, there is no guarantee that the OE setting are still the best.