In the Canadian Army, the PB&J is a go-to option for food that can be eaten while standing in a crew commander's hatch of an armoured fighting vehicle, navigating, and monitoring the communications nets. You're monitoring the nets because talking with a mouthful of sandwich will make it hard to get your message across. Good thing you have a signaller to respond for you.

Variations on the PB&J include the Body-Builder Sandwich, which is Cheez Whiz and peanut butter on raisin bread, a favourite of mine when I was a gun position officer on recce in winter.