A vacuum gauge is also an early warning device of a tight valve, worn/leaking seat, intake leak - the needle will fluctuate at idle. A constant slightly low reading at cruise may mean your vacuum advance isn't coming in on time, point setting slipped etc. - not enough spark. I used to have a favorite test road - a long slightly uphill with no traffic and mile posts. I'd check the gauge at the same air temperature, gear & MPH after every mod (including oil viscosity, tire pressure, alignment), if the vacuum was up I'm doing the right thing.
