Bottom line, a belt pouch is period correct for the 18th century and one can use it for what one wishes.
18th century Rifle Matches were often one shot matches, though they may have shot in more than one match in a day.
Modern ML matches often have at least two or three, if not five or more shots per match. Some matches include starting a fire w/flint and steel and if it is too hot for a waistcoat or jacket, a belt pouch can be the perfect place to carry those "fixins" or anything else one might wish.
For those who like "minimalist" or "day" pouches, a belt bag can be just the ticket for carrying extra things one needs.
Gus