In addition to coming to Jem and Scout's rescue and saving their lives (it is a crucial part of the story, after all), Boo serves as the character who connects the two main plots together at the end. The mystery surrounding Boo in the first half of the novel shows the children slowly understanding that Boo is not really a bad guy after all. This transition serves to illustrate one of Atticus' famous sayings--
"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."
Boo is also a symbol of the children growing up, realizing that the rumors that they had heard for so long about Boo were untrue. Boo is also one of the human mockingbirds in the story--an innocent man accused of terrible things. Boo's rescue of the children from the murderous hands of Bob Ewell connects the second plot of the story--that of the Tom Robinson trial--when Boo kills Bob, Tom's false accuser. It marks the completion of the transition from Boo the Ghoul to Arthur Radley the Hero.