As Scout learns that it's not OK to judge people on the basis of color alone, so does the reader...
Her limited perception and understanding also limits the reader and is sometimes confusing.
The advantages to having a young character as narrator are that Scout's innocence highlights the social prejudice, injustice and racism in the story. Scout cannot initially appreciate the fear and rejection which surrounds Boo, the mockery of a trial where a clearly innocent man is found guilty or the harshness of a society who accepts a man be punished not for a crime but for the colour of his skin. Scout has to 'learn' the prejudices of the society and the complex social codes which operate. Her innocence magnifies the cruelty in the systems which she is brought up to understand as the principals most adults live by.
As Scout is actually telling the story as an adult recounting her childhood, there is a maturity of observation and reflection by the end of the novel which guides the reader through the simplistic child view. Also, Scout's advanced education and literary prowess make her a very articulate child narrator. There seems no real disadvantage as a result.
Advantages: Scout is a child when these events take place, so the reader sees the events through her eyes--not colored by the bias and prejudices that the rest of the town seems to have. She simply reports the events and tells the reader her understanding of them, rather than telling the reader how the reader should think. Telling the story from Scout's point of view can also make the story for poignant and allow the reader to have greater sympathy for Scout and her lack of understanding.
Disadvantages: Events and people may not always be clear to some readers. A reader had to "read between the lines" in order to understand what Scout is reporting. Also, some people may have a problem with Scout reporting such "adult" events, but I feel it makes it all the more touching.
Making the narrator an adult recalling her childhood point of view is crucial. It makes possible the intermingling of our still-forming, childish, magical view of the world and an "objective" view of the world, which as adults (hopefully) we learn is just as hard to pin down. In my childhood, there was treasure at the bottom of every lake I swam, when i was being punished, my parents were secret agent imposters and the customs of other households were as if from an exotic foreign land. The world feels this way for scout. Boo's house, the rabid dog, the treasures in the cigar box, as well as the fascinating behavior, good and bad, of the adults around her have that delicious, terrifying magic tinge. She is able to watch from the sidelines and then, when she is attacked in the woods, she is forced to participate. Her view is a perfect metaphor for the perpetual, never quite coming of age story of mankind with its fear, cowardice, heroism and magical thinking.