I was told: It keeps the ball & powder from rolling out of the barrel.
A holdover from the olden days. These days there is no chance of that happening.
Placing a firearm which fires from an open bolt muzzle down is not a good idea. One bump and you could have an AD. Ever notice that soldiers always jump off of a truck or obstacle with the muzzle up if they are holding a firearm that fires from an open bolt? Bad idea to do so with the muzzle down. Gravity and inertia are not your friend.
A plugged barrel will always have the potential for harm. Muzzle up is just all around the best idea.