I think the primary function of the lube is to keep the black powder fouling soft allowing you to shoot longer and more easily between cleanings.
If you are cleaning after each course of fire it's probably not necessary.
It might help cut down on leading.
It probably helps protect against chain fires, though I suspect the tight-fitting ball does most of the work there.
Period pistol cartridges had lubed bullets but it was a greasy wax in the bullets grease grooves. They did not cover the bullets over with lube nor use a wad.