The first person point of view of Harper Lee's To Kill a Mockingbird is perhaps one of the most effective techniques she uses to further her purpose. Scout's perspective is one of innocent storytelling, calling things as she sees them, and very often getting them wrong. It is humorous and yet very honest and refreshing. Lee is able to convey the hypocrisy, prejudice, and lack of justice in the 1930's south in a way that paints America's history into a shameful memory.
If the story had been told by anyone other than Scout, it would lack almost all of its uniquely ironic humor. Also, Boo Radley might not be such a prominent character. Additionally, the reader would miss some of the best lessons and quotes of Atticus, which are universally applicable even though they are applied to an innocent child in the story.
From a completely personal perspective, Scout is one of my favorite narrator's in all of literature. I'm sure many others feel the same way. If this book was told by someone other than her, it might not be so popular nor so well loved a story.