Why did Atticus want Mr. Ewell to write his name during the trial?

Atticus wanted Bob Ewell to write his name to show if Bob was left or right handed.  It was important because Mayella Ewell had a black eye and bruises on the right side of her face.  This would indicate that she was hit by a left-handed person.  If Mayella and Bob were facing each other, a right-handed man could not have hit her on the right side of her face.  This fact was significant because Tom Robinson’s left arm was useless and deformed because of an accident he had earlier in his life.  It proved that Tom Robinson could not have hit her like Mayella and Bob claimed.  It was a great defense for Tom; unfortunately, the jury had already made up its mind about Tom’s guilt.