One must consider that priming before loading the charge was the method taught by the military from the invention of the side lock muskets until the caplock obsoleted them.

This was done due to the issue paper ctgs.

The making of a French Ctg. for the musket.

English Ctgs.

Note, the American Army issued both buck and ball as well as single ball. Buck and ball loads used, more than doubled the numbers of those with a single ball.
When cap-locks obsoleted the flinters, the cap (priming) was placed on the nipple after the load was rammed down WITH 3 blows of the rod on the seated ball.

We try to discourage people from doing that today, but many still do. Throwing the rod on the ball damages the ball, meals the powder changing it's burning rate and has the potential to detonate the powder if the barrel is hot already. Piston fire starters are an example. Apparently once at a Southern US rendezvous, a man's rifle discharged on the 2nd or 3rd 'throw'. Now, did the percussion/compression of air/powder from the rod on the powder ignite the powder, or an ember still there, from a wimpy load, we'll never know. I do know or a fact, many fellows South of the border use too thin patches and leave a dangerous discharge possibility unless wiped after every shot.

Old habits die hard, good or bad.

Today, we do not allow priming before loading the gun, for obvious reasons.