This is a widely misunderstood topic. And the plumbing for the vacuum source is not a tell - so it is not as simple as "on the carb" versus "on the manifold". Many carburetors provide full manifold vacuum from a fitting on the carb base that is below the throttle plate. The term 'ported' vacuum refers specifically to a vacuum fitting that is above the throttle plate.
To FTFs point the the vacuum advance is designed to advance spark timing when the charge is lean - a condition found at idle or when cruising on the freeway at part throttle. Manifold vacuum signal addresses both driving situations. "Ported" vacuum only addresses the latter - and was an early emissions fix that was coupled with completely different centrifugal advance curves and initial timing settings.
A great article on this topic can be found here:
Chevellestuff 
