Although it's small the power it contains is life saving, literally. Without it, the fairy cannot live her normal life. Her normal life is helping mortals, without that she does not want to live. Once it became dim and the jewels, which decorated it, were no longer beautiful, it still has power hiding behind it. To anyone else, it would be insignificant. To the fairy, it meant the world. It either meant her life would be back to normal, or turned into a life of misery and despair. Even the tiniest objects can have the biggest impact.