the game gives it to you and says it's special, thats all, it's not any different from a regular weighted cube in terms of properties. But the game keeps referring to it, (especially in the hidden survivor rooms) and gives you the impression that's its special for some reason. If you think about it the walls, the turrets, the cameras, and the cubes of aperture science they are all sterile and uniform but then the game introduces a unique cube and you latch onto it. A lot of portal is mystery and dark humor and the companion cube is just another dark joke in this excellent game.
If you wanna get analytical about it I suppose you could say that it's mocking games that arbitrarily give you a love interest who is aggravating and useless. You have to drag this character around keeping them alive just so you can get a stupid kiss in the sunset or some other bullshit like that and you end up hating the person because they are getting in the way of playing the game. While in portal they give you an arbitrary love interest with no back story, no voice, and no gender (no heart beat either) and ask you to use it to block deadly plasma balls and provide dead weight on a button to open a door, so it's funny because it's like telling someone to become companions with a door knob or a spoon.
