Roommate 1: "I found an unused, extra toothbrush!"
Roommate 2: "Keep it for drunk guests who can't find their toothbrush during State Patty's weekend."
Roommate 1: "It's one of those things that would be useful if only you can find it when you need it."
Roommate 2: "Yep, it's one of those usefulless stuff..."
Something that's really really really useful. You don't know how your grandparents survived before it came along. When your one blows up over the weekend you love paying for the after-hours call out fee because you can't face another day without it.
This planet is a usefulnessity. I would die without it - or float around in space until I died.