Why does Fitzgerald begin this book with a description of the party at the Buchanans in East Egg?

We really learn about the lives of the filthy rich. Most of this money, unlike Gatsby's, is old money. It is money that has been inherited over generations. The people are class oriented and have very little understanding about how most of America lives. Nick Buchanan, for example, is immensely wealthy. He is a socialite, a hypocrite and sexist. Nick notices that these traits seem to be almost prerequisites for males of the highest class.