Harper Lee had a personal relationship with courts and courthouses. This biographical fact seems to go a long way to explain the detail of the courthouse description.
Also, this setting is one of the most specific and important in the novel. Tom Robinson's trial is the crux of the novel's action.
In addition, the trial scenes highlight the difference between the travesty of the very real events taking place in the courtroom. These events will, without a doubt, finish destroying Tom Robinson's life. And yet many of the community at re following the trial merely for the entertainment value. Something so sinister being used for the amusement of many, displays one of the weakest characteristics of the old south.
One of the reasons for the highly realistic and detailed courtroom scene during the trial of Tom Robinson was author Harper Lee's own experience with the law. Her father was a small-town lawyer, and Atticus' character is based on him. Lee herself went to law school, and left just six months short of graduation. The trial was the centerpiece of the second part of the novel, and Lee had slowly built its anticipation during the earlier chapters. It was an important event in such a small town--a case that could result in the death penalty for the accused. The supposed crime itself--the rape of a white woman by a black man--was bound to enflame the white population of the town, and Lee wisely chose to treat the trial with the proper intensity. The trial is considered one of the finest examples of courtroom drama in all literature (as is the film version), and Lee's attention to detail allows for the reader to piece together the individual facts and decide for himself who the real guilty party is--just as Scout does.
Lee's description of the courtroom and the circus-like atmosphere pervading the Tom Robinson trial underscored several things about the setting of To Kill a Mockingbird. First of all, the Greek Revival columns evoke memories of the architecture of the Old South and the plantation aristocracy, a lifestyle that has been gone less than one hundred years, leaving behind attitudes and prejudices that will still require years and years of struggle to even begin to overcome. Secondly, there wasn't much going on in Maycomb in 1933; early in the novel, Scout observes that due to the Great Depression, there was nothing to buy and no money to buy it with, so the scandalous nature of Bob Ewell's accusations against Tom Robinson and the black against white nature of the trial provided free entertainment for the whole community.