The introduction or conclusions to an essay on symbolism in this novel is a good place to discuss what the use of symbolism adds to the book, the effect it has on deepening the meaning of the narrative and the ways the book's symbols serve to a unifying function in the text.
In her use of symbolism, Harper Lee manages to convey the moral message of the book through this artistic device. This is especially true in her treatment of the novel's central symbol - the mockingbird.
As the title of the novel implies, the mockingbird serves as an important symbol throughout the narrative.
Not only are characters like Boo Radley and Tom Robinson aligned with the symbol of the harmless mockingbird, but other characters are as well. Atticus, a figure of virtue and moral power, can also be seen as a harmless figure, in need of protection.
By aligning a number of characters with the mockingbird symbol, Lee effectively creates two camps or groups within the novel. There are people who strive to do good (the mockingbirds) and those who they need to be protected from (much of the rest of the town: Ewell, Mrs. Merriweather, etc.).
This crafty unity of characters around a value-oriented symbol serves to add power to the narrative. This is perhaps one reason behind the words of the Pulitzer Prize committee of 1961:
"...first of all it is a story so admirably done that it must be called both honorable and engrossing."