Harper Lee’s To Kill a Mockingbird is a coming-of-age story, also known as a “bildungsroman.” In this kind of story, the protagonist, in this case Scout, is shown to grow and develop as a person.
Part of Scout’s growth in the novel is spurred by her father’s role in the trial of Tom Robinson, the black man wrongly accused of raping the white woman Mayella Ewell. Thus the trial itself, and Atticus’s role in it, is an important plot line in the story.
Lee emphasizes the trial’s importance as Atticus begins to address the all-white jury. The narrator describes Atticus as he steps out of character:
Atticus did something I never saw him do before or since, in public or in private: he unbuttoned his vest, unbuttoned his collar, loosened his tie, and took off his coat.
This is a big deal for Atticus, a bastion of stability and good sense, yet he suddenly changes his approach as a lawyer. It tells the reader that this trial is something above the ordinary, and that it will have an important impact on the rest of the story.
Then Lee employs a common literary device, the pun, to emphasize the central focus of the trial, when Atticus says to the jury:
This case is as simple as black and white.
In the early 1960s, when Lee published the novel, this statement was considerably more controversial than it is now. Of course, what he means is that the defendant, Tom Robinson, a black man, is the subject of white prejudice. Lee has set the novel in the 1930s, when prejudicial attitudes were much more prevalent than they are now.
This shows the reader how important Atticus believes the trial to be. It is an important moment when somebody steps out of character. Writers use such points in their stories to signify watershed moments in the narrative.