The importance and function of Boo Radley in Harper Lee's To Kill a Mockingbird is that of serving as a point of comparison of the usual behaviors of the people of Maycomb concerning things, or people, that they neither know nor understand.
It is through the town's reactions toward Boo that we can sense a tendency to give unknown people, or unknown things, a quality of mystery, enigma, and even lore. We also see how the people's perceptions of what they do not even know spread from household to household, strengthening the validity of what it simply is a rumor.
Miss Maudie, for example, tries to explain to Scout that Boo is just a normal person. However, Scout is not buying it
Arthur Radley just stays in the house that's all. Wouldn't you stay in the house if your didn't want to come out?
Poor Boo is a lonely man with what seems to be limited intelligence. He likes kids and leaves them small, insignificant, but well-meant gifts just to see them smile. However, he is a recluse in his own home. This is what makes rumors run amok. Yet, like Tom Robinson, he is never given the chance to be judged by his qualities as a human being. In Atticus's own words:
What Mr. Radley does is his own business. If he wanted to come out, he would. If he wanted to stay inside his own house he had the right to stay inside free from the attentions of inquisitive children.