Answer and Explanation:
1. The director wanted to show how profound Atticus' final argument was and how it had a great meaning in his life. Through the close-up, the viewer can have a detailed view of Atticus' expressions. This makes the viewer have the impression that Atticus is talking to the viewer, that is, Atticus is presenting to the viewer a delicate, deep and very impacting subject.
2. Atticus' facial expressions and body language show how angry he is at having to participate in something as unfair and deplorable as racism and the lack of justice in a court of law.
3. If the scene were filmed at a wide angle, the viewer would not be able to get the feeling that they were in a conversation with Atticus. Thus, the viewer would not feel that he could be part of the injustices caused against blacks due to racism, as the angle would present a tone of remoteness and impartiality.
Importantly, the movie "To Kill a Mockingbird" is based on the book of the same name, written by Harper Lee, which features a black man who was accused, without evidence, of raping a white girl. Atticus, a highly respected white man, becomes this man's lawyer and even if he has proof of his innocence, he can't defend him from the racist, prejudiced, and intolerant society.