Why does Nick include Gatsby's story of his courtship of Daisy, the information about his war service and his return to Louisville after the war, at this? What does it reveal about both Gatsby and Daisy?

This reveals that Gatsby was not always this aloof god of riches that people see. Like countless of men, Gatsby was a man of meagre means who was in Love. Daisy was and is a self serving opportunist. Even if she did have feelings for Gatsby, she chose the way of easy money and an abusive marriage of convenience.