In this passage, Scout is standing on Boo's porch, and she begins to imagine the way Boo must have seen the events that unfolded in the novel. For example, in the summertime, Boo sees the children (Scout and Jem) coming closer to his house. In the fall, the children stop at an oak tree (the tree where Boo placed their presents) and examine it with delight. In summer, Boo sees the children's hearts break during Tom Robinson's trial.
Scout says at the end of this passage, "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." She recalls that Atticus told her that a person can never truly understand another person without walking around in the other person's shoes. This is the key to empathy--to truly understanding the way another person thinks and feels. By standing on the Radleys' porch, Scout begins to understand the way Boo experiences the world. She has truly developed a sense of empathy and can see the world the way Boo sees it.
The author uses third-person because the novel is narrated by Scout as a woman looking back on her childhood. There is one section in Chapter 31 where it seems as if Scout is narrating in the first and third person. Scout is there on Boo's porch, recounting the events as if they happened to other children. This section where Scout recounts the events of the novel is structured by the seasons: summer, winter, summer, autumn. The author does this to acknowledge that Scout has been recalling the entire novel from the perspective of an older woman.
Most importantly, Scout recounts these events in a third-person narration style to reinforce the idea of standing in another person's shoes and looking at things from a new perspective. This is one of the lessons Atticus teaches in the children. Fittingly, Scout is on Boo's porch and sees her street from a new perspective. Likewise, the events of the novel have passed and in a kind of out-of-body experience, as if she is watching herself go through the events in summary, she now looks back on them with more wisdom and understanding.
It is to reinforce the idea of standing in another persons shoes and how a shift in perspective can change a great deal.