I don't know anybody in BPCR competition who does NOT use an over powder wad of some sort. I just use the Walters "vegetable fiber" wad, whose markings indicate that they are punched out of gasket material. Some use HDPE wads. Some use two wads, a card wad over the powder, a wax paper or newsprint disk over the card to keep it from sticking to the base of the bullet in case you did not get all the lube off the bottom.
I wouldn't bother with sixgun shells but it is pretty much a standard for black powder rifle ammunition, especially if it is to be shot at any great range.
The purpose is usually said to be to protect the base of the bullet from impact of powder granules and flame. The base of the bullet being the "steering end" and a clean exit from the muzzle important to accuracy.