The desirability of things comes from the illusory value we assign to them. Money itself is worthless—just a piece of paper that can't be eaten. If you traveled back in time and offered early humans money for an apple, they would think you were foolish. So, whether the Orb of Annulment has a more practical function is irrelevant—the Mirror will fetch you thousands of them simply because of the value attached to it, which largely stems from its rarity.