Earlier in his testimony, the sheriff said that Mayella Ewell was bruised on the right side of her face. This means that someone who was left handed probably hit her. During his testimony, Atticus has Mr. Ewell write on a piece of paper. He uses his left hand to hold the pencil. Atticus then has Tom stand up and show that his left hand is useless since he injured it when he was a boy. This makes Mr. Ewell a prime suspect in Mayella's beating.
Oookay, that's not an answer.
The real reason it's so significant is because it's a real example of how racist we were, how unfairly they were treated, and their reputation in society. Tom Robinson had a perfect case going for him, but the jury decided to vote in favor of Ewell because of his race.