Interesting this should come up. I just got two. Found one at Sportsman's Warehouse and got a slightly different one from Campmor. About $5 each.

Kinda like the old GI canteen cup. Fits over your water bottle so it's out of the way but there when you need it.
And one might need it if spending an unexpected night out with just daypack gear. Can't heat water in your Nalgene bottle and warm water sure would go nice with your survival fire wouldn't it? (and you can melt snow, and you can make soup or tea or ramen or.....)

unimogbert