One that comes to mind was the manuals and bits and pieces that came with Infocom text-based adventure games. They were nice bits of cruft to have alongside the actual game but in certain instances puzzles within the game could only be solved by referring to something on the card, or booklet, supplied with the game. I can't recall if it was The Hitchhiker's Guide, or Leather Goddess of Phobos, but the requirement popped up quite deep into the game.

They weren't license keys, persey, as all the printed material was the same, but a tacit test as to whether you had bought the actual game, or just copied the disk.