What is special about chapter 20? Harper Lee's To Kill a Mockingbird

Not only does Chapter 20 of To Kill a
Mockingbird present Atticus Finch's summation to the jury, it also presents a
synopsis of many conflicts that exist in the society of the small town in Alabama in the
1930s:

man vs.
society

• Mr. Dolphus Raymond, a
  man of the social upper class who prefers to live with the blacks on the "wrong side of
  town," reveals further to the children that he actually drinks Coca-Cola from the bottle
  wrapped in a paper back.  He tells the children that he does so in order that the
  gossips can feel justified in misjudging him.


• Tom
  Robinson has committed the grave social blunder of feeling sorry for a white person. 
  Along with this grave faux pas,  he is perceived as lying since his
  testimony contradicts that of a white.  Atticus alludes to this when he questions in his
  summation the conventional wisdom that "all Negroes
  lie."


• Later in Atticus's summation, he reminds people
  that "all men are created equal" under the justice system of the United States.  They
  might not all be equal in the realm of education, but they certainly are in a court of
  law.

individual vs. the
stereotype

• In the South, the
  woman of the 1930s was put upon a pedestal.  Certainly, Mayella Ewell is at the nadir of
  such a lofty ideal as she goes against all social mores when she lusts after a black
  man.  So, she tries to cover this blunder by accusing Tom of rape.  But, Atticus refuses
  to excuse her behavior simply because she has lacked opportunities for a decent life. 
  He urges the jury to  consider the code of their society in reaching their verdict as
  Mayella acted as she has in the courtroom to cover her social
  sins: 

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"No code mattered to her before she broke it, but
it came crashing down on her
afterwards."

good
vs. evil

• In his summation,
  Atticus reminds the courtroom of the evil that lies beneath all societies and suggests
  that the rules of society are probably such in an effort to control this underlying evil
  in humans.  Pointing to the connection between man's evil nature and the rules of
  society, Atticus contends that it is guilt that motivates Mayella to charge Tom Robinson
  with rape.  He reminds the courtroom that there is not a person who has not told a lie
  or a man who at some time has not desired a
  woman.

Replete with conflict, this chapter of
Harper Lee's novel also repeats the theme of intolerance as
Mr. Raymond consoles Dill who cries about the events of the trial.  He explains that
Dill will not cry in a few years, but now as children they will "Cry about the hell
white people give colored folks."  In addition, the theme of education
emerges as Atticus speaks of the equality of people in a court of law. 
He says that all are not equal when it comes to education, and people who profess that
they are operate under a terrible delusion.