So in those cases where manifold vacuum at idle is not beneficial ( including not having the proper canister for the application such as with engines that have low unsteady vacuum at idle) then ported vacuum may used to achieve nearly all the benefits of a vaccum advance.
In many situations the reason manifold vacuum is helpful is the mechanical advance. Using manifold vac is a way to crutch it and much simpler and easier. It does require a stable and reasonable strong vacuum at idle.