Scout's standing on the Radley porch after having walked Boo Radley home metaphorically fulfills the maturation theme initiated by Scout's father in Chapter 3 when he instructs her,
"You never really understand a person until you consider things from his point of view--....until you climb into his skin and walk around in it."
Now, after all that has transpired, Scout realizes that she and Jem and Dill have not reciprocated for all that Boo has done for them--she considers things from Boo's perspective.
Atticus was right...you never really know a man until you stand in his shoes....Just standing on the Radley porch was enough.
As she walks home, Scout remarks that she feels "very old"; indeed, she has matured as she stands on the Radley porch. When she arrives at her house, her father has taken up The Gray Ghost, the third of a trilogy by Seckatary Hawkins, a book that contains an incident in which there is a character called Three-Finger Pete, a character not unlike Boo Radley. Ironically, Jem has won this book by going up to the Radley house, and now Scout returns from having stood on this very porch. This scene, too, ties together the entire narrative as it connects with one of the early chapters, both relative to the Radley porch. In Chapter 1 the children are engaged in a childish prank; in Chapter 31, Scout steps off the porch feeling "older" with new insights on life.