What is the importance of Lee's minor characters?

In To Kill a Mockingbird, racism and tragic injustice take center stage against the backdrop of life in a small Alabama town during the Great Depression when Scout and Jem are growing up. In Scout, Jem, and Atticus Finch, Lee creates unforgettable major characters who are caught up in Tom Robinson’s being accused of rape, brought to trial, and convicted of a heinous crime he did not commit. Secondary characters—Miss Maudie Atkinson, Atticus’s sister Alexandra, and the elusive Arthur “Boo” Radley—are drawn into the narrative in ways that advance the plot and develop several subplots in Scout and Jem’s journey out of childhood. Maudie, Aunt Alexandra, and Boo are unforgettable, too.
The major and secondary characters and their storylines are more than sufficient to drive the novel, but To Kill a Mockingbird does more than develop Harper Lee’s plot and subplots. It also captures the culture of Maycomb, Alabama, in the 1930s and records the rhythms of daily life. Through an amazing, diverse collection of minor characters, Maycomb comes alive on the pages of the novel, and human nature is illuminated in ways that inspire hope for humanity or cause anger and despair in regard to how terribly people can treat one another. Sometimes the minor characters simply evoke laughter as they demonstrate how silly human beings can be.
Many of Lee’s minor characters are either “black or white”—but the distinction exists in regard to their humanity, not their race. The goodness and evil in Maycomb, as in the world at large, have nothing to do with the color of one’s skin. Heck Tate, Judge Taylor, Link Deas, Dolphus Raymond, Reverend Sykes, and Calpurnia are all examples of goodness. Each of them acts with courage and kindness rooted in moral conviction and concern for other human beings. Compassion and respect for justice are evident in how Heck, Link, and the judge deal with Tom Robinson’s incarceration and trial; Heck’s staunch refusal to subject Boo Radley to public scrutiny in regard to Bob Ewell’s death demonstrates compassion and respect for justice, as well. The sheriff defies Atticus and bends the law he is sworn to uphold because he cannot tolerate the moral injustice that would occur if he acknowledged Boo’s having killed Ewell. In Dolphus Raymond, Reverend Sykes, and Calpurnia, the reader finds loyalty and love. Each of them, at various times and in their own ways, reaches out to the children with tenderness because they understand and care about the difficulties Jem, Scout, and Dill are experiencing.
Evil in varying degrees is found in Bob Ewell; Nathan Radley; Scout’s teacher, Miss Gates; and Grace Merriweather of the Missionary Circle. Bob Ewell’s accusing Tom Robinson of rape, thus deliberately destroying Tom’s life, and later attempting to murder Jem and Scout are the ultimate acts of evil among the minor characters. The racism of Miss Gates and Mrs. Merriweather is not manifested in violence, but it is hateful and destructive. Jem’s brief encounter with Nathan Radley illustrates an evil not motivated by racism, but it is enormously cruel. As Radley methodically fills the knothole in the oak tree with cement, he deliberately destroys Boo’s communication with Jem and Scout, the only source of joy in Boo's life. When Jem realizes why Nathan Radley cemented the tree, he recognizes the evil act for what it is and cries.
For most readers, the minor characters in To Kill a Mockingbird are memorable personalities in the novel, even if their names are forgotten. Sometimes they are remembered as the no-nonsense housekeeper who made Jem and Scout behave or the hypocrite who hated Hitler or the judge who sat one night reading a book with a shotgun in his lap. That the minor characters are remembered even when their names have been lost is a testament to how vividly Harper Lee brings them to life. Without them, much of the truth about human nature and life in a particular time and place would have remained unexplored. As they are woven into the tapestry of the narrative, however, To Kill a Mockingbird becomes an even richer, more satisfying, and more illuminating novel. The minor characters, it seems, aren’t minor at all.