Most times when mages play it against me it has a neutral outcome, or it screws them over. I've had one or two games in which it won the game for them, but mostly it sucks. I don't know why anyone would include it in their deck.
Because it's fun. It causes things to happen in games that you cannot predict and moments that are unpredictable. Not everybody wants to win at all costs all of the time and box has given me some hilarious or very cool moments in games that a majority of cards simply can't give. Since regularly using a quest, discovery mage deck I've had multiple friend requests after games with people complimenting the deck and saying what a cool game we had, win or lose.
A majority of players aren't competitive players. They aren't hitting too 100 and going for tournaments etc and so sometimes they just want to have fun. Hanar and box are two of the most fun cards I've ever used. Sure, box does me over as often as it helps me but I don't play this deck because I'm expecting a steady, consistent strategy. I play it because it's the backend of an expansion following a rotation and so there are the fewest amount of cards available and I'm a bit bored. My games rarely feel similar and I have no idea how they'll go, this keeps it fresh for me at the moment. I have no interest in grinding ladder, I use my janky mage deck to get to around diamond 3 and it never feels a chore.