how her father even knows she is there. It is much later that she finds out her father wanted her to overhear his words. He wants Scout and Jem to speak openly to him about any questions or concerns they have regarding the trial. He does not want them to listen to idle gossip around Maycomb. He prefers they communicate openly.

I believe that Atticus allows Scout to purposely overhear his conversation with Jack for two reasons. First, Atticus has not found a way to sit down and explain to his children about the upcoming trial and the implications it will have on the family and community. By frankly discussing the Tom Robinson trial with Jack, Scout is able to hear Atticus's side of the story and his own reasons for taking the case: It is in part because Judge Taylor appointed him to do so, but he also took it because of his children.

"... do you think I could face my children otherwise?"  (Chapter 9)

(Oddly, Scout must not have understood her father when he explained to Jack that Judge Taylor told him " 'You're It.' " Scout only learns this later when she hears members of the Idlers' Club discussing it.) The second reason Atticus allows Scout to hear his conversation is to show her how proud--and forgiving--he is of her recent behavior. Scout has been forced to listen to her classmates ridicule Atticus for being a "nigger-lover," and to her cousin Francis claim that her father is "ruinin' the family." Scout defends her father by fighting Francis, but is in turn spanked by her Uncle Jack for doing so.

"... I hope and pray I can get Jem and Scout throuugh it without bitterness... I just hope that Jem and Scout come to me for their answers instead of listening to the town. I hope they trust me enough..."  (Chapter 9)

One reason is that he wants to chastise her for her "hotheadedness" because Atticus is worried about what is coming up. He uses his conversation with Jack to voice some concerns; he hopes that Scout will pick up on them, and take them more to heart than if he had given her a direct lecture.  Since Scout has already lost her temper twice over even the mention of Atticus and his trial, he is worried that "Scout'd just as soon jump on someone as look at him if her pride's at stake".  He hopes that Scout can get through the trial "without bitterness...[and] that Jem and Scout come to me for their answers instead of listening to the town.  I hope they trust me enough."  He is telling Scout that she is going to have to be calm, steady, and come to him if there are difficult times, instead of fighting or endorsing other people's beliefs.

His conversation with Jack is a kind of preparation for Scout, to bolster her for what is around the bend.  He is concerned, and that worry spills over in this conversation.

Miss Maudie indicates that Atticus is "civilized in his heart" and didn't want "an unfair advantage over most living things", and so he doesn't boast about his talent.  He realized it was a gift, and to brag about it would just make others feel bad, so he doesn't.

In his conversation with his brother Jack, Atticus is frank about how difficult life's going to be for the Finch family once the trial of Tom Robinson gets underway. Atticus wants Scout to hear every word of this conversation, as he wants to prepare her for what's coming next.

He knows that this is a much better way of telling her than giving her a stern lecture. As Scout is so stubborn and headstrong, there's a real danger that she wouldn't take in everything that Atticus said if he talked to her directly, whereas if Scout overhears her father say the exact same things to Uncle Jack, it's more likely that she'll take his words to heart.

Atticus wants Scout to be calm and sensible during the forthcoming trial, and instead of giving her direct orders, he wisely thinks it's much better if she learns how he wants her to behave by hearsay.