The rule is designed to prevent collusion or chip dumping. Also, in a tournament the other players not in the hand have a vested interest in your elimination (unlike a cash game, where only your opponent in the hand at the time really cares). This way, you can't just muck your cards at the end, everyone gets to see what you went all in with-- theoretically preventing you from dumping your chips to a friend.
In a cash game it doesn't matter. If you wanted to give chips to someone else, you simply could do it in the hallway.
