Preparations are just pre-meditated Advantage. When something lets you make a Preparation, you can set up a specific trigger or plan at which point you can invoke it freely for Advantage1 when it’s relevant.
You don’t need a Preparation to claim Advantage from some plan; it just makes a clear signal for some plan you’ve set in motion2 in advance.
Unlike Quests, I don’t think these are just another type of Motif… though I don’t know why it couldn’t be.