Lee told the story of the novel through the first-person eyes of the young girl Scout Finch for several reasons. First, the novel has autobiographical elements. Creating a narrator just slightly younger than she herself would have been at the time allows Lee to accurately record the feelings and details of the Depression era as she experienced and felt them as a child. This lends an authenticity to the work that might otherwise might be missing. By leaning into her vivid and precise memories of the South in the 1930s, Lee is able to make Maycomb seem an utterly realistic place.
Second, one of Lee's goals is to show Atticus as an exemplary human being. As the writer Jane Austen noted, "pictures of perfection" in literature can make us sick, so Lee had to find a way to show Atticus' positive traits without putting us off by depicting him as overly saintly. Having him portrayed through the eyes of an adoring but at the same time feisty, frank, and tomboyish little girl who is not easily impressed allows Lee to interject humor and make Atticus palatable to us.
Finally, Lee wanted to show the injustice of the racist system of justice in the South at the time. Using an innocent child who was not trained to accept the racism she sees in the courtroom, Lee is able to frankly expose the unfairness and cruelty of the system through the eyes of one who sees and tells it as it is without adult rationalizations.