I'm running an early iron vac advance dizzy on one motor and a later aluminum one on the other, so they do exist (yes, they really are advance, not retard). The later one is a Euro model; I don't know if they were used in the North American market.

Both are hooked to the full manifold vacuum, so they advance with closed throttle and retard under wide open throttle. The centrifugal advance still increases with rpm, as expected.

The point of this is to have more advance under conditions where the cylinder is only partially charged, because that's a slower burn rate than you get with a full charge. Power is unaffected (at WOT there's no vacuum advance at all), but fuel economy improves a lot in normal driving.