Why does Atticus make such a big deal about Mr. Ewell being left-handed? To Kill a Mockingbird by Harper Lee

In the trial scene in Part II of To Kill a
Mockingbird, Bob Ewell's left-handedness is the smoking gun that implicates
him in the beating of Mayella.  Atticus gets Bob to write his name, and the jury notices
that he's left-handed.  Atticus says that since Mayella's bruises were on the right side
of her face and that she was choked with two sets of good hands around her gullet, Bob
Ewell is her attacker and not Tom.

Atticus' clever
tactics should lead to a not guilty verdict from the jury, since Tom's left hand has
been crushed in a machine.  But, alas, racism makes the jury blind to the
facts.