The narrative structure of Harper Lee’s To Kill a Mockingbird allows it to look at the process of growing up and how our understanding of the world changes during childhood and adolescence.
Using Scout as a narrator frames all of the story’s events within the mind of a six-year-old. At the same time, we know that Scout is actually an adult looking back on events in the past. So everything she tells us passes through two layers of perception: that of the child, and that of the older and wiser adult.
As an adult, Scout is able to look at her childhood actions and beliefs ironically—she now knows how wrong she often was as a child. This mirrors society as a whole—Maycomb county is struggling with the Depression and sometimes violent racism. This society, like Scout, needs to grow up and learn to treat others appropriately.
The reader sees how Scout has grown (even though she is now just a year or so older than she was at the beginning of the story) by a comment she makes in the last few pages:
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough.
Scout has learned a lot. Childhood and adolescence should be full of such experiences and revelations. By putting the story in adult-Scout’s hands, Lee enables the reader to see how she has grown. By extension, it also shows us how society as a whole should be growing.