What is the purpose of telling the Finch family history at the beginning of the novel?

Harper Lee's inclusion of the Finch family history at the beginning of her novel serves several purposes. Not only does it give the reader a sense of the known heritages the Finches can claim, but also illuminates their social class and status within the town, and even within the region.
Not only does it display this to the reader, it also clearly shows what Scout, as the narrator, knows about her family history and how the Finch family is regarded. She's been taught, by members of her extended family, that she needs to take pride in her heritage and her Aunt Alexandra expects her to live up to these demands. This high sense of propriety is in contrast to the casual way that Scout addresses her father by his first name, Atticus.
On an even larger scale, Lee uses this family history to put this story in a very specific time and place. It does not begin in a "Once upon a time" land or have nameless characters. These are specific people who lived in a certain town during a specific era. These are all intentional choices, as the rest of the story cannot hang together without these aspects. When it is all put together, the history makes the people, and the story more real, as none of it is perfect or glossed over, but all of it relatable in some way--no family is perfect, and no history is perfect either, even Aunt Alexandra's Finch relatives.

At the beginning of the novel, Scout elaborates on her family's history. This is significant to the exposition of the story and reveals important background information about her family's status in the small town of Maycomb. Scout discusses how her ancestor, Simon Finch, immigrated from England and established a homestead named Finch's Landing on the banks of the Alabama River. Following the Civil War, the Finch family lost much of their wealth but managed to save their homestead. Though Atticus chose to travel to Montgomery to study law and his brother moved to Boston to study medicine, Alexandra remained on Finch's Landing, where the family gathers each Christmas. Atticus ended up moving back to the small town of Maycomb, where he became a well-known lawyer. Scout's family history not only identifies their revered social status in the community, it also corresponds to Aunt Alexandra's obsession with heredity. Scout's description of her family's history explains Alexandra's affinity for heredity and sense of familial pride. One of the main themes Harper Lee explores throughout the novel is social class and status. By elaborating on Scout's family history at the beginning of the story, Harper Lee subtly begins to emphasize the themes of social status and class.