Why is lighting lamps (Deep Daan) so important on Kartik Poornima?

During the entire month of Karttika, lighting ghee lamps for Lord Vishnu is the most meritorious act. On Kartik Poornima specifically, the Gods themselves light lamps to celebrate the awakening of Lord Vishnu and the victory of light over darkness. A lamp lit for the Lord on this night is said to grant worldly prosperity, remove darkness from one’s destiny, light the path for ancestors, and ultimately lead the soul towards Vaikuntha.