She shows different perspectives. This allows the reader to enter the minds of the characters and try to see things from their point of view. Atticus himself says that you never really get to know a person until you have had a chance to step in their shoes. This is an important point not just because it directly relates to the story, but because it is universal. Therefore, anyone can read the story and find something to relate to, because all views are represented. Examples include Miss Maudie's common sense and liberal attitudes, Aunt Alexandra's stuffy ways mixed with good-hearted intentions, the obvious love and loyalty beneath Calpurnia's outward toughness, etc.
Sometimes a character's situation is explained to the reader through another character. An example is the Cunningham's unconditional refusal to accept handouts. The Cunninghams never say so themselves, of course, but the reader learns of their ways and values through statements made by Scout, Calpurnia, and Atticus (the syrup incident, the entailment, etc).