A good question. Aunt Alexandra is having friends over. Notice how everyone is on their best behavior - from Scout\'s dress to Cal\'s starched apron. This chapter is designed to show the reader the importance of one\'s family and name in Maycomb. This means that people are not separated just by economic means or social standing but also by their lineage (though one could argue that is directly linked to economic means and social standing).
Alexandra hopes to show Scout what it really means to be Finch. As such she has certain expectations to live up to. And behaving like a dirty little tomboy is not one of them! Acting like a respectable young lady who knows with whom to associate (and the Cunningham boy, who is considered white trash, is not one of them).
This chapter also illustrated how important family and their standing among Maycomb\'s citizens really is to Alexandra. Atticus could care less. Scout realizes this too. Notice on page 233, Scout says \"I was more at home in my father\'s world.\" This is a reference to how Atticus treats - or tries to treat - everyone equally. Scout has inherited that trait.
Remember if it were up to Alexandra, Atticus would never have taken the Robinson case. Defending an African American accused of rape was not the type of think a respectable Finch did.
Part of the priorites of the people of Maycomb is a sense of history. Harper Lee begins the book by describing the history of both the town and the Finches. Having a family background - meaning knowing the history of your family, both good and bad, and embracing it - gives people a sense of belonging and a place in the world. Scout, Jem, Miss Maudie, Attius, even Calpurnia all benefit from a strong family history. Dill, on the other hand, is from a disjointed family with little history and suffers as a result.