Why does atticus ask Tom Ribinson to stand?

Atticus asks Tom to stand to illustrate to the jury.... and the other present, that Tom could not have committed the crime of which he is accused.

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 was 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 hand, and from as far away as the balcony I could see that it was no use to him. "