What is Lee's purpose in having the Ewell family accuse Tom Robinson of rape?

I think the fact that the jury would never take the word of a Ewell against another white person speaks volumes. The town is fully aware of the Ewell family and their lifestyle. They are scorned and shunned. The glaring irony is that the all white jury of men will still take the word of a Ewell over the obvious truth of a black man.