How it works:
When much is required of the engine, it is more likely to ping (poor mixture) than if the engine is running effortless (rich mixture). If we give more pre-ignition to an effortless running engine, the efficiency will increase and the engine will use less fuel.
The vacuum in the intake manifold is a measure of the load on the engine. (floor it up the mountain = gas valve open -> little underpressure. Cruising down the mountain = gas valve partially closed -> a lot of under pressure/vacuum). This means that if there is a vacuum in the intake manifold, we can advance the ignition a little.
Why you'd want this:
In summary: through the use of vacuum advance we can decrease the fuel consumption, the combustion will be cleaner and the efficiency will improve.