What can the reader infer about Lee's purpose for introducing Uncle Jack in this chapter?

Uncle Jack is sort of an intermediary between Atticus and Scout. He is level headed like Atticus but his relationship with Scout is a little different. While Uncle Jack doesn't understand everything about Scout, he does have a a certain amount of objectivity that Atticus might not have as her father.

Uncle J. seems rude... no... nice. He should know everything about Scout. He's her uncle!