As well as going through the actual events of the alleged rape, Atticus in this chapter also, as Scout remarks, is 'building up a picture of the Ewells' home life'. This is almost as crucial as determining the actual details about the attack on Mayella because, by highlighting the dismal and sordid conditions of Mayella's home, he also highlights the fact that Mayella is actually an unreliable witness who lives in fear of her father.
By a series of questions, Atticus elicits from Mayella such details as the fact that she lives a life of drudgery, has scarcely been to school, and has a tribe of younger siblings to look after as well as doing practically all the chores around the house. In all of this, he draws out the essential fact that Mayella is a very lonely individual who has no-one to turn to. This comes to light when he asks her if she has any friends. She seems frankly 'puzzled' by such a question, and then lashes out:
'You making fun of me agin, Mr Finch?'
Atticus let her question answer his.
What this means is that Mayella is so friendless that when asked if she has any friends, she automatically assumes that she is being mocked. From this Atticus is able to lead up to the most important point of all:
“Do you love your father, Miss Mayella?” was his next (question)
“Love him, whatcha mean?”
“I mean, is he good to you, is he easy to get along with?”
“He does tollable, ‘cept when—”
“Except when?” Mayella looked at her father, who was sitting with his chair tipped against the railing. He sat up straight and waited for her to answer.
“Except when nothin‘,” said Mayella. “I said he does tollable.” Mr. Ewell leaned back again.
“Except when he’s drinking?” asked Atticus so gently that Mayella nodded.
By way of skillful questioning, Atticus is able to catch Mayella off her guard and admit that she is vulnerable and that there is a problem with her father. When he asks her about his father's drinking, he does it so 'gently' that she seems almost unaware of what he's asking and automatically comes out with the truth: that there have been occasions when her father has been less than 'tollable', or tolerable, to her. In this way Atticus is trying to show the judge that Mayella's own father, not Tom, could have been her attacker.
Of course, she goes on to retract this and to say 'firmly' that her father has never attacked her, but,thanks to Atticus's subtle and gradual approach, not a little doubt has been cast on her testimony that it was Tom who attacked her.