When I was very very short my family would spend time over at the Davis’s place three or four times a year to load shotgun shells (which, really, was an excuse for adult companionship, food, and a couple of beers). At some point during the evening we short people would beg to be given a job, and my father and Mr. Davis would task one of us with setting either the copper primers or the plastic wad into the machine.
When I was much taller I was introduced to black powder, where you put this greasy waxed bit of patch between the powder and the ball.
I’m pretty sure that when I was nine years old I asked what the wad was for and was given the answer, but now I can’t remember and the people I could ask are dead and/or gone, so…
Yeah, I get that in regards to shotgun shells, but in muzzle loading rifles it takes a bit of elbow grease to get the lead down the pipe. Is the “seal” between lead and iron really so bad that it needs a bit of cotton for a guarantee?
The wad gives you a bit of wiggle room when you’re making your own balls. When you roll your own, it’s easy to make them a little bit too small but a bit harder to make them too large (underfilling the mold is a lot easier than overfilling it). With those irregular ball sizes, the wad is pretty necessary.