The crowd that attends the trial in Chapter 16 of To Kill A Mockingbird represent all the types found in Maycomb County.
There are the Mennonites (rarely seen in town); the foot washers ("a wagonload of unusually stern-faced citizens"); the high-class ladies (represented by Miss Stephanie Crawford, wearing her hat and gloves, but denying that she is going to town to see the trial); the poor whites; the "Negroes"; suspected drunk Mr. Dolphus Raymond with his black wife and his mixed children; and "the Idlers' Club ... a group of white-shirted, khaki-trousered ... old men ... attentive critics of courthouse business."
As can be seen from this list, the author, Harper Lee, accomplishes many things by including all these people.
First of all, she gives us a richer picture of Maycomb County. This is the only time, for example, that the Mennonites appear at all. The parade of people gives Jem an opportunity to give commentary on their colorful histories to Scout, by which we readers learn about them too.
Secondly, the presence of some of these people allows the author to explore in more detail the dynamics of race in Maycomb County, and Jem and Scout's growing struggle to understand the social "rules" surrounding race. This happens, for example, when Jem points out Mr. Dolphus Raymond's children to Scout. She asks, "How can you tell [they're mixed]?" and Jem replies, "You can't sometimes ... but around here once you have a drop of Negro blood, that makes you all black." Later, Scout overhears the Idlers' Club's commentary on Atticus and the trial, and is once again confused by their racist attitudes. Later still, we see the Idlers' Club try to prevent the black people from entering the courthouse. Scout and Jem ultimately end up sitting with the blacks in the balcony.
Finally, the presence of every group shows what a big deal this trial is to all the citizens of Maycomb. It is rare that such a sensational event comes along. So a few people are there just for the spectacle. But most people are there because the results of the trial are important to them. A black man has been accused of raping a lower-class white woman, and a highly respected white lawyer is going to do his utmost to defend him. In this trial, the blacks, the poor whites, and the upper-class whites all feel that they have something to lose. To say nothing of the families of the accused, of the alleged victim, and of the lawyer.
The fact that all these people from different walks of life, all of whom have competing stakes in the trial, are brought together in one place, makes the tension very high in Chapter 16.
It is only natural that all of Maycomb would turn out to see this trial, because it is a defining moment for Maycomb. If Tom Robinson is acquitted, it will forever change the status quo in Maycomb, and change it very dramatically. But even if Robinson is not acquitted, the status quo will still be shaken because Atticus will have shown that Robinson patently did not do it. The Cunningham family will lose face, and the upper-class white citizens will have to grapple with the fact that their system is unjust, and to face their fear that one day the status quo will change forever.
In short, no matter what the outcome of the trial, everything changes, and everyone loses something. It is appropriate that all the citizens of Maycomb should be there to see their world change.