As Danwagon mentioned, the dual vacuum cannisters typically hooked the ported source to the advance and the retard side was hooked to manifold vac. It was done to reduce emissions and raise idle temperatures. It also depends on your model of distributor and car. A vacuum switch tee was used as another emission control device. It was used in conjunction with the dual vacuum advance canister on the distributor. While the engine was warming up, it sent full manifold vacuum to the retard side of the distributor canister. This retarded the spark advance resulting in higher combustion temperatures which had the effect of warming up the engine faster and burning some of the otherwise unburned hydrocarbons during the warm-up period. Once the engine coolant was warm, the vacuum switch closed off the manifold vacuum port signal at the tee, and very little or no vacuum was sent to the retard side of the vacuum canister.
Depending on how your distributor and or vac can is set up, you might find that your car idles better with the advance side hooked up to a manifold source and the retard side disconnected.
Doug
