It happens often: you're running around with your portal gun, holding onto the companion cube for dear life while hundreds of rounds of bullets bounce off your friend as he protects you. You see your destination and realize there's a half dozen turrets pointed in that direction. You sprint forward and accidentally throw your companion cube, or drop it. GAME OVER. But the beauty of having the companion cube is that it's tough, it's durable and it's always there with you, even after a test gone horribly wrong.
You're running to class, work, or some place where you have to bring your own lunch. There are no turrets, no incinerators, and no giant pits, but there are other people, the sidewalk, and cars. Someone bumps into you and you slip, launching your precious Portal 2 Companion Cube Tin Lunch Box skyward. Don't fret, it's made of durable tin and as it meets the concrete, it may get a bit scratched up, but its contents are safe and sound and ready to be picked back up and loved, at least until you gnash them with your teeth.