I agree with post 6--that the trial represents baby steps in the moral change coming in Maycomb. Atticus Finch himself knows that the task to defend Tom Robinson is difficult and Tom likely will be convicted. Atticus takes on the task anyway, and proves to the watchers at the trial that Tom is innocent using the evidence at hand such as Tom's crippled hand. The town knows, but most cannot acknowledge that Tom has told the truth about the Ewells. The long wait for the jury's decision, the newspaper editor being willing to listen and change, and the townspeople truly listening, all indicate that change is coming. The trial reflects the town's attitude to those watching, and a few don't like the view they see of themselves or their town. Slowly, change in attitudes and actions will come to Maycomb.
The trial matters to Maycomb as a whole because of the sensationalism that any serious trial brings to a small town. Small towns thrive on gossip and small talk for entertainment, but a trial, especially one as sordid as the Ewell/Robinson case, brings a whole new level of entertainment for the locals as well as new fodder for the gossip mills.
The trial brings into question the standing social order of Maycomb as it regards race. There is a prevailing attitude, borne out by the trial, that a person of one "color" cannot successfully win in a case going up against a person of another "color".
Should this premise of justice be altered, the social order of the town would be altered and that is no small thing.
Some of the people were there just to get revenge. They honestly thought Tom Robinson had defiled a young white girl. Some of them were there just to see racism continue unchanged. There were a very few, like Atticus, who were there to see change.
The outcome of the trial takes Maycomb "a baby step" forward into the 20th century. Tom's conviction was to be expected, but the fact that it took hours for a jury decision to be made by the all-white jury, and that racists like newspaper editor B. B. Underwood could be swayed to defend Tom show that times are changing in Maycomb--though at a snail's pace.
One reason it is important to the community, so important that they all gather there, is that it represents a battle between ways of believing and living. As such, people want to witness the battle waged between moral right and moral wrong, truth and falsehood, freedom and oppression and they want to see who the victor will be: moral Atticus or immoral Bob Ewell.
The trial is important to Maycomb as a whole because it may set a precedent, many citizens feel. If Tom Robinson is acquitted, Tom Ewell and his daughter will be placed in a compromising position in the white/black social order, and a ruling in Tom's favor will, Maycomb citizens probably think, empower the Negro populace of the town, giving them a "voice" in their lives.
As an agent of change for Maycomb, we have to consider the trial to be a failure in many ways. Tom Robinson is found guilty, and institutionalized racism appears to maintain its hold on the South.
Harper Lee provides a glimpse of hope by showing how some of Maycomb's citizens manage to keep a fair and impartial perspective, but for now, at least, there aren't enough of these people to bring widespread change to the community. Lee is telling us that it's really tough to make such changes, and that we have to find a way to treat each other right in the face of an injustice that we might not be able to defeat.
For me, it's important because it will sort of determine whether Maycomb's old social order will continue. Under the old order, blacks are always subjugated, whites are always on top. An acquittal would, in a way, undermine that order. So the trial is going to determine whether Maycomb stays as it is or becomes more modern and more just.
The trial shows us how deeply corrupted the legal system is in Maycomb when Tom is still convicted although there was irrefutable evidence that the Ewells themselves were not innocent.