In my opinion, the Ewell family helps demonstrate the need for equality. It demonstrates that no matter what the color of a person, there is the chance for evil to come from anyone.
The Ewell also helps the reader feel sympathy for Tom Robinson. When we know for sure that Mayella Ewell has so many reasons for accusing Tom, (none of which have to do with the truth) and when we see that Bob has every reason to present good reason for Mayella's injuries that someone must have seen, we empathize with Tom Robinson. The sense of injustice makes certain that readers will side with Tom. Thus, when we learn of the verdict, it makes white readers mad at a white family. This is good to help drive home the point that criminal activity, lies, and injustice are colorblind. Therefore we must seek equality for all.
To Kill A Mockingbird exposes the small-town mentality of Maycomb County and its inability to move away from its prejudiced views. Reputation, real or imagined, and the protection of it, contributes to the ready acceptance of the townsfolk to Tom Robinson's guilt. Keeping up appearances is apparently far more important than anything meaningful. Atticus teaches his children not to judge others and certainly not to draw conclusions "until you climb into his skin and walk around in it" (chapter three). He is not like most of the residents of Maycomb, namely, the "foot-washing Baptists," even though some of the Finch family, as Scout relates in chapter one, feel "a source of shame" because they cannot trace their ancestry sufficiently far back enough.
The Ewell family, "the disgrace of Maycomb for three generations," who are, ironically, despised by the townsfolk, must protect Mayella from any impropriety. Her father believes that any interaction between Mayella and Tom must have been uninvited, and he intends to lay all blame (for what he perceives) squarely on Tom because his actions must have been grossly inappropriate, as far as he is concerned.
For Mayella to be seen treating a black man with any respect would be shameful and unacceptable (and the reason why Bob Ewell beat her senseless and then blamed Tom for assaulting her). Unfortunately, most of the townsfolk see the support of the Ewells as less improper than the defense of an innocent man who is black. Even those who do not believe Bob or Mayella's story do nothing to stop the injustice for fear that they might ruin their own reputation. Even when it becomes apparent that Bob Ewell, Mayella's own father, beat her, it still is not enough to save Tom; saving face for the townsfolk is far more important.
Through To Kill a Mockingbird and Bob Ewell's accusation against Tom Robinson, Harper Lee has revealed the senselessness of this damaging, shortsighted, unreasonable attitude and the travesty of justice which led to such a devastating end for Tom. She has also shown how even indifference can have such dire consequences.