I don't believe it has anything to do with shooting or blackpowder. The shot pouch/flask heads have a notch for the retaining spring, and a horn has two ends to plug. A 1/4" opening (+/-) for the spout and a base plug - which is mechanically attached with pins, pegs or nails. A loose "chained" lid would spill everytime you moved; and when your life depends on dry powder you want a waterproof and solid/secure closure.
I agree with the chain retainer - but likely the other end was a toggle to hold it in the inkwell or Pullman coach brass sink.