I mean that's the whole idea, you don't create knowledge out of nothing. But it's more than just preexisting belief, you try to grab it from collective unconscious.

Plus, card on itself doesn't tell you anything SPECIFIC. The whole point of cards and magic is to believe that this specific card has bigger meaning, thus you need to interpret it, and think about it's meaning. And thinking is useful. What conclusion you come up with depends more on the situation than the card itself.