Are you on step no. 2348 on your way up to High Hrothgar? You have a long and cold journey ahead, so bring out that metal mug you hooked to your backpack and pour yourself some Firebrand Wine. Will you, like The Dragonborn, be out and about in the mountains in the near future? Then you need a mug that you can easily bring with you. The metal mug has a carabiner to be hooked to the backpack. Imagine how nice it would be to pour up something tasty when your horse is stuck in a particularly steep place as it's not really as good a climber as the ones from Tamriel.