Fitzgerald includes the list of guests that attend Gatsby’s party for two reasons. First, it pokes fun at the epic tradition of cataloguing, providing grand and detailed lists of battles, weapons, soldiers, etc. Secondly, and most importantly, it paints a great picture of the type of life Gatsby is immersed in. Like East and West Egg, the novel explores the huge divide that exists between new and old money. This subtly seen but extremely important class difference is ultimately the reason why Daisy and Gatsby cannot be together. Unlike the prestigious and elite East Egg, West Eggers are new money—they often lack class, poise, and sophistication. Gatsby’s guest list includes these new moneymakers, people that came to wealth not through family and linage, but through get-rich-quick schemes, racy Broadway shows, and sleazy “coneggtions” through the likes of Wolfshiem. Names like “James ‘Rot-Gut’ Ferret’ hint at the shady nature of these characters. Like Gatsby monstrous mansion, the parties may appear beautiful at a glance, but upon further inspection, it is all simply smoke and mirrors—a carnival scene without real substance that even Daisy eventually recognizes
Gatsby spent his whole life trying to attain wealth and prominence in society, and through that, win back Daisy, the love of his life. The catalog of Gatsby's guests is included to show how far his star has risen. He invented himself, but not only did he believe it, but so did the prominent guests who came to his parties. It is one of the rare cases, in this novel, when the old rich (those like Daisy) actually long to do something associated with the new rich (like Gatsby).
Gatsby, who is new money, is constantly trying to affect the the ways of the old money, a theme that is constant throughout the whole novel. Just as Gatsby tries (and fails) to adopt the mannerisms of the old aristocracy, so too does he attempt to talk like them, thus using the phrase "old sport."