Boo Radley is essential to the book "To Kill a Mockingbird" because he is a catalyst for many other things to happen through out the book. For example he allows the reader to see what the mentality of some of the people in Maycomb. For example Miss. Stephanie Crawford, who demonises Boo as a monster and also a large proportion of the community who, for example, will not eat the nuts produced by the tree on the Radley's property as they will be poisoned.
"Miss. Stephanie Crawford said she woke up in the middle of the night one time and saw him (Boo) looking straight through the window at her" Boo Radley also aids the relationship between Jem, Scout and Dill as they are united with one main objective, which is to make Boo come out.
Harper Lee uses Boo to show the reader the gossip and attitude of the towns-folk towards people, or things, they do not understand.
This can also be applied for the prejudice in the town towards black people using Tom Robinson.
As the reader starts to move through the novel we can see where the title came from ("To Kill a Mockingbird"). Harper Lee chose this title wisely as it is a metaphor for the theme of the book. She chose this name from a passage that Atticus said to Jem when he was given a rifle: "Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird". He is referring to the notion that a mockingbird is a harmless creature that does nothing all day long but "sing its heart out" to bring happiness to the world. At the end of the novel Scout says "to tell about this (killing Bob Ewell) would be like...