In the cafeteria at work there are many varieties of milk, differing in their fat content: skim, one percent, two percent, whole, and half and half. It's annoying to keep all of them in stock with all their different expiration dates and speeds of use. So why not make a machine that holds skim and half-and-half, and can dispense the desired ratio? Something like a standard milk dispenser but with two bags and a valve.

As a bonus, for a given amount of milkfat mixing your own right before you drink it tends to taste richer.