For performance reasons.
Running full vacuum @ idle is not really ideal.I always use ported vacuum.
For instance, lets say you have 40 degrees @ idle, as soon as you start giving it some throttle the vacuum will drop causing the timeing to retard, then when you let off full advance, then on the throttle, timeing retards again, it goes all over the place.
Sure, there is mechanical advance also, but initially, when you give it throttle it retards, then the mechanical advance will kick in more advance later.
You can set your distributor w/more initial advance & limit you mechanical advance ( so you do not get too much total advance)
You will have greater throttle response, as long as you can get away w/more initial advance.
Hope that makes some sort of sense?
My two cents. :-)
