Tighter emmisions standards

From what I have gathered approx 1972 tighter emmissions went into affect, one of the changes made was to reduce the amount of initial advance at the dampner. In some cases from what I have read these slant sixes could have been set at 0 or even BTDC from the factory to get this lesser emmissions reading at idle.

I do not understand how this made the engine opperate more efficiently or helped with fuel burn-off.

From the way I understand it we tend to want to advance our vehicles as far as is possible ( without pinging ) so that the fuel sprayed into the cylinder has a better chance to be utilized. Cleaner more efficient burn.

Were the manufacturers just oblivious to how an internal combustion engine operated back then or were there other emmissions items added to the vehicle to offset any negative condition this poor initial advance produced