mostly it was for emissions. it helps a little with a 73 equipped with EGR.
basically it has the extra feature of retarding timing. so for a boss it help it meet cali emissions requirements being hotter exhaust temps at idle.
for 73 cars with EGR they also equiped the dual vacuum again for more emmisions the exhaust recycle causes a hicup in the vaccum of the engine which messed with the automatic transmission modulator so to compensate ford installed a dual vacuum modulator on the transmission as well.
you can remove it and go back to a single advance and you can also install a 71-72 modulator if you want. it is kind of like the same argument with distributors with Dual points verse single points. the dual points keeps things in line at high rpms but its complex to set dwell on and maintain it.
