What pertinent fact did we learn about Tom Robinson's physical condition?

In Chapter 18, we learn that Tom Robinson's left hand is useless.

Tom Robinson’s powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly off balance, but it hand, and from as far away as the balcony I could see that it was no use to himwas not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled .