In To Kill a Mockingbird, one reason why it makes much more sense for Harper Lee to use the unreliable narrator Scout rather than an omniscient narrator concerns the direct parallel Harper is trying to convey between Scout and the South, specifically the small Southern town of Maycomb.
Scout is a young and growing girl. Similarly, the South has undergone so many transformations, particularly having to transform from a slave territory to a free territory, that it proves to be still quite immature in this time period with respect to its racial relations. The immaturity of the South is portrayed in its Jim Crow laws, its racist attitudes, and its hypocrisies, all despite the fact that it is also the Christian Bible Belt of the country. Like a petulant child, it refuses to let go of old, racist ideas and embrace new ideas. Yet, at the same time, there are those who display the character traits of Atticus who help the South embrace new ideas and grow as a region. Just as Scout learns and grows from Atticus so does the South, specifically the tiny town of Maycomb. Hence, Lee uses the unreliable narrator Scout, complete with all of her growth and development as a character, to parallel the rising, needed growth and development of Maycomb.
Just as the citizens of Maycomb have generally been raised to exhibit Christian Values, Scout has been raised by Atticus to exhibit Christian values, such as kindness, generosity, and forgiveness. In fact, Lee frequently uses illusions to portray Atticus's Christian ideals and to relate him to Christ. For example, in Chapter 9, Atticus confides in his brother Jack about how poor the prospects are of being able to acquit Tom Robinson and how he had always hoped to be able to escape having to deal with a case so full of racial bigotry, saying, "You know, I'd hoped to get through life without a case of this kind, but John Taylor pointed at me and said, 'You're it'" (p. 91). Jack's response is to indicate Atticus's suffering by making a biblical illusion to Christ in asking, "Let this cup pass from you, eh?" (p. 91). More importantly, the illusion also serves to set Atticus up as a Christ-like figure and a role model for Maycomb, as well as the rest of the South, to follow and learn from. Hence, just as Scout has a Christ-like role model from whom she is learning so does Maycomb, and Scout's unreliability as a narrator who is learning and growing serves to parallel Maycomb's own unreliability as it learns and grows.
Scout is actually the best narrator for the story because she is innocent, still learning about people/society, and herself. She is very intelligent, is a good girl, and she really doesn't fit in to the southern society that she calls home. She is not afraid to question and does a good job of making the adults in her life think about their own actions. Scout is a tomboy who really isn't afraid of much, so , because of these qualities and the fact that she sometimes feels like an outsider herself, she is able to tell the story in her own unbiased way and it allows us to see how the adults in her small town handle the conflict, as well as relationships.