What do you learn from Bob Ewell's evidence? Is it that Mayella was abused and harassed?
After Mayella Ewell accuses Tom Robinson of raping and attacking her, Tom is brought to trial with these charges and assigned an attorney--the very intelligent and capable Mr. Atticus Finch.
When Mayella's father, Bob Ewell, is put on the witness stand, Atticus asks him to write his name down in front of the court. Bob Well confidently does so on the back of an envelope, which reveals one critical piece of information to the entire room: Bob is left-handed.
This information is so important because Mayella had come away from the "attack" with her right eye blackened. Logically speaking, a left-handed person would have to have been the one to swing from the left in order to hit the right side of her face. After Atticus reveals that Tom has a crippled left hand from an accident with a cotton gin, the court can logically begin to doubt that he was the perpetrator of the crime, directing them back to the truth: that Bob Ewell beat his own daughter up.
Probably the most important thing we learn is that Ewell is left-handed. Whoever hit Mayella Ewell on the night of the alleged rape was also left handed. The bruising on her face, especially on her right eye means that someone was leading with their left hand. Tom Robinson has no use of his left hand because he got it caught in a cotton gin when he was a boy. Therefore, the most likely person who hit Mayella was her own father.
Ewell is left handed and whoever hit her face must also be left handed. Tom could not use that hand because he was maimed working on a cotton gin as a boy. Therefore, the only likely answer is that Bob Ewell punched Mayella in the face. Atticus announces this information through the trial that causes Mayella to become nervous.