Alternating between the child Scout and the adult Scout, the reader is given a dual perspective. The effect is that the reader is presented with a child's point of view of events as they occur and an adult's point of view of events in hindsight. Given that the adult's point of view comes decades after the events occur, the reader is given more wise, thoughtful, and reflective perspective. Whereas with young Scout, the reader is given a more reactive and innocent perspective. Therefore, the reader gets the wisdom of the adult Scout but also the naivety and innocence of the young Scout.
Another effect of this technique is that the reader begins to understand, throughout the novel, how the young Scout became the adult Scout; in other words, how the innocent Scout learned about the world and became the wiser adult. This is significant not only because of how the technique makes the reader consider both perspectives but also as it relates to one of the themes of the entire book: considering the perspectives of others.
This is one of Atticus' main teaching points: to always put yourself in another person's shoes. Jem and Scout slowly learn this over the course of the events in the novel. And in the last lines of the novel, Scout realizes this in more ways than one while standing on Boo Radley's front porch:
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.