I think that Harper Lee includes the two impoverished families to illustrate that while one might be impoverished or part of a society's lower class, he does not have to be classless. Bob Ewell is obviously classless and raises his children to have the same lack of work ethic and ethos. Mayella carefully plots to entice a married man and then frames him to save herself. Similarly, her little brother Burris is truant and thinks nothing of insulting an authority figure.
In contrast, the Cunninghams--while also being members of Maycomb's poor, struggling class--demonstrate class (for the most part). Mr. Cunningham cannot pay Atticus with money for his legal assistance, but he is careful to pay in whatever manner he can (food goods, etc.). Walter, Mr. Cunningham's son, has obviously been raised with the same sense of diligence. At the dinner table, he converses with Atticus about his hard work in the fields with his father, and Walter possesses such a sense of personal dignity that he is unable to explain to Miss Caroline why he does not have a lunch.
Thus, while both families suffer from the same physical effects of the Depression, they are completely different in not only their sense of decorum and decency but also in how fathers influence their children negatively or positively.
The Ewells represent the lowest white class of society. Lee frames this class as people who won't work, and who are stuck in a cycle of self-defeat and welfare status. The old English Ewell actually is evil.
The Cunninghams are the next highest class of society, willing to work, but poor and refusing of charity.
These classes exist to show that there are different types of people morally and according to work ethic. Later in the book Scout and Jem analyze this and come to the conclusion that there should just be one type of folks. Folks.
The Ewells, and the Cunninghams are both part of the lower class of Maycomb. However they portray themselves differently . The Ewells lack a sense of care, education, and jobs. But the Cunninghams try to make money the best way they can, and also encourage there children to attend school.
Thank you for answer, it helps me too!