Why does the narrator give so much information about her family and background in the beginning of the story?

The first chapter's emphasis on family history and stories within stories describes the rigid social ties that hold society together in the little town of Maycomb, Alabama, and the inescapable links that tie an individual to his or her family or clan. The book opens by mentioning how at age twelve, Jem broke his arm. The narrator notes that the remainder of the book will explain how this injury occurred, and the novel concludes with this event. From the outset, through historical analysis, the novel tries to conclude how "this particular situation" arose. The children's attempt to trace the main incident in the novel (Jem's broken arm) back to its roots, leads them to wonder whether it all began when Dill first arrived in Maycomb and became their friend, or whether the real origins lie deeper in their ancestral history and the chance events that brought the Finch family to Maycomb. Their debate speaks to deeper fundamental issues on the nature of human good and evil, and the old "nature vs. nurture" debate. Dill, the new kid in town, represents an outside influence upon the children that affects them deeply, whereas the family history Scout recounts is a more inexorable pattern which existed long before the children were born. Atticus tells Jem and Scout that patterns of history, family, identity, and temperament, both new and old, help make an individual.