Atticus gives hope to the town of Maycomb because he is the town’s moral center. When no one else will do the town’s dirty work, he does.
First of all, Atticus is chosen to defend Tom Robinson because he will do a good job. He is one of the only men in the town respected well enough.
"We're the safest folks in the world," said Miss Maudie. "We're so rarely called on to be Christians, but when we are, we've got men like Atticus to go for us." (ch 22)
It was no accident that Judge Taylor chose Atticus Finch. He knew that he would do a good job, and he would be strong enough to stand up to town ridicule.
Second, Atticus defends the town from the mad dog. Symbolically, this is how he defends the town from racism. When a rabid dog comes down the block, the sheriff Heck Tate tells Atticus he needs to be the one to shoot, because it’s a one-shot job.
"You haven't forgot much, Mr. Finch. They say it never leaves you." (ch 10)
When everyone else runs and hides, they call on Atticus Finch to do their dirty work.
Finally, Atticus is able to get the town to think about racism, and consider rejecting it.
[He's] the only man in these parts who can keep a jury out so long in a case like that. And I thought to myself, well, we're making a step- it's just a baby-step, but it's a step. (ch 22)
Even though Atticus does not get a not guilty verdict, he does manage to get the jury to actually deliberate.