I've been playing Judgment for a few days and, much as I'm enjoying it, it's driving me nuts that I can't figure out the purpose of the key "minigame." It says to choose the matching key, but there's nothing to match against! Am I missing something? Why is this mechanic in the game??

I'm not stuck btw, I don't even think I've guessed wrong yet (purely by luck), but...just what is the purpose?