My wife and I make custom cabinets for customers. We work together on almost all of the fabrication but she takes over on the sanding and finishing. ( I really miss not getting to do the sanding... Yea right!). She insists on using sanding sealer on every project. I question it but she keeps saying that if I can get a better finish, help myself! She feels like the sanding sealer makes a better base for the top coats. She sands the sanding sealer lightly before applying the top coats. Customers rave about the finishes. Who am I to try to fix something that isn't broke? Besides, I might have to do all that sanding!
I think it is one of those preference things that if it works for you... use it.
Don Dando