Distributor Vacuum Advance?

Adding to what Dave said... the vacuum advance adds timing when the engine doesn't mind it. This gives (in most cases) a decent jump in gas milage, keeps the spark plugs cleaner longer, and means the engine is more powerful at partial throttle. It has no affect on idle quality, nor does it do anything at wide open throttle. If you have a race car, or typically don't spend much time at part throttle like on the street, or have an engine combination that doesn't have a vacuum signal in the range of the vacuum pod's adjustment, then running without one makes sense. Otherwise, hook it up and set it up and it will only help.