why do dist. have the vac advance and the mechanical advance in the same dist?

It is a part-throttle load compensating device to give you better performance/gas mileage.

The "perfect timing" setting changes with rpm, throttle position, and load. The centrifical advance is strictly rpm based and doesnt take into account load or throttle position. Vacuum advance couples adding or removing timing based on load and throttle position. They affect timing at any throttle opening BUT idle and wide open. Mopar vacuum advances (and fords from the little I know) are also partially tunable. The vast majority of street cars that don't run vacuum might benefit from using a properly set up one. Once the camshafts get "so large"(something like 260°@.050 with stoke strokes and 9-10:1 static ratios would be too large.) the vacuum signal changes enough that the vacuum advance cant really be ajusted to work with it. At that point it can create issues and should be removed/plugged.