Answer:
It expresses the central theme of the entire poem. The concept of lost beauty over time and demonstrating what the depiction of this missing thing is. They are compared to something that everyone envies and wishes they had. Showing that even though there are grains of gold scattered throughout life and passage, they will never be permanent. You will never be able to keep that gold since it will eventually deteriorate. Nothing significant, fools gold, could be represented since if a golden moment remains, you know it was never real.