Early muzzle-loading firearms required the user to carry all the gunpowder they’d need themselves loose in a simple container like a powder horn. With early-modern storage methods being considerably less reliable than modern techniques, you needed to be really careful to not let the gunpowder get rain on or submerged since that’d make it pretty useless.
So, keeping your powder dry means being ready for action.
Yeah unfortunately Andrew Jackson’s still on money so…