This short and sweet trick tutorial is packed with valuable insights and principles. It's also a rant about how weird it is that magicians get coins signed. It's bizarre; why would anyone ever sign a coin?
Now, I understand why a magician might encourage a spectator to sign a deck, though it's usually unnecessary. With a card to an impossible location, you might want a way to confirm the card that vanished is the same as the one that appeared on the other side of the room. Or that the fixed card is the same as the one you restored.
It makes sense to do so on one level because it's your card, and it's easy to find an identical playing card from another deck, plus THE ONLY EXPLANATION FOR BOTH OF THESE EXAMPLES IS THAT YOU SWITCHED THE CARD.