Lee uses contradiction to point out how severely embedded racism is in society in Maycomb when Tom Robinson turns out to be crippled.

It is not accident that Lee waits until the trial to tell us that Robinson could not physically have committed the crime.  This contradiction of Bob Ewell’s testimony is very dramatic.

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. (ch 18)

Robinson contradicts Ewell’s testimony because Ewell argued that he attacked his daughter.  Atticus proves that there is no way Tom Robinson could have produced the injuries that Mayella had.  Instead, he implies that Ewell himself hit Mayella when he found her with a black man.  Tom Robinson never raped Mayella.  She kissed him, and that was it.

This contradiction serves to enforce the severity of the racism in Maycomb.  Even though he could not have committed the crime, Robinson is convicted.  This huge injustice infuriates young Jem, and reminds readers that racism is wrong.

Contradiction can be a powerful tool.  The reader expects Tom to be acquitted, but he isn't.  We feel the sting of injustice along with Jem, and at the time the book was written people were beginning to think about these things.