This is funny because it involves loading revolvers.
I load several cylinders at a time off the frame using a loading press.
I drop in the powder charges and then add a wax board paper wad over the powder and then ram down the powder and wad before any ball is placed on top of the paper wad.
Wny? Because the APP powder compresses better that way and I'm sealing the powder charge with the wad.
Only after the powder is compressed do I place the balls on each chamber and ram them individually. At this point, the powder can be slightly additionally compressed by the 2nd ramming to the depth and amount of my choosing.
I've found that APP greatly benefits from high compression.
I believe that Pyrodex P is another powder that also benefits from high compression.
If a person wants to maximize their powder compression without needing to place as much effort and stress on the loading lever or on their hand while loading a revolver, then I would recommend tamping down the powder in this manner.
It helps to maximize the amount of powder that can be squeezed into a chamber with a finite dimension.
And high compression is what allows 46 grains or more of Pyrodex P to be loaded into a Remington 1858 chamber.
It may or may not work the same for black powder since it doesn't compress in the same fashion as Pyrodex or APP or perhaps Black MZ powder as well.
But when you have a very coarse powder like Black MZ I don't see how it can hurt to ram the powder into the chambers individually in order to maximize compression and effort with a short loading lever, depending on the revolver and the length of the loading lever.
Let's not characterize all of the loading methods into the same basket, and realize that some unconventional methods can have a logical reason and benefits to them.
Another example I was just reading about was when Val Forgett went on safari in Africa, and he loaded 200 grain powder charges into a .58 Hawken Hunter and killed hippo, elephant and buffalo using over 600 grain conicals.
I can certainly understand the reason why a person loading 200 grains of powder might want to tamp down their powder charge before they ram their 600 grain bullet.
I'm not saying that he did or he didn't, but who wants to argue that someone's loading procedure is better than another's.
That what makes muzzle loading an individual shooting sport.
And also why revolvers are in a category among themselves.
If it can work for a revolver, then perhaps it can work for a rifle.
I'm not one to argue, but mainly trying to stress a point while trying to objectively answer the OP's question which was: "Is there a reason or an advantage to tamping the powder before you add in the bullet?"
I suppose there could be a reason or an advantage.