Suspense! During the mad dog scene, we see Atticus using a skill that not many know he possesses. We watch the dog slowly turn back up the street and begin walking toward Atticus. Of course Jem and Scout have no idea what Atticus is doing (since they think he is more of a coward than he is), and we wait for the moment when Atticus finally pulls the trigger and drops the dog.
We have the same type of suspense during the trial. Atticus has displayed his lawyer skills to the best of his abilities - in fact, has even proved the Ewell testimonies false. We wait on the verdict because we know Atticus is right, but we also know that it would take a very strong show of bravery and conviction on the part of the jury to allow Tom to walk. Although we don't see the same success with the trial that we did with the dog, we do see how Atticus is able to stick true to his character and teach his kids a valuable life lesson.