Atticus makes the jury feel bad, and want to take Atticus' side, the audience is engaged in how Atticus is speaking, and acting, he is making a good case, and the counter-argument is weak
Atticus shames the audience and members of the jury whose prejudice has taken control over their thoughts in such a way that some were unwilling to even consider the evidences and realize that they were about to send an innocent man to jail. The audience is very engaged during Atticus' speech and he wants everybody to take his side, obviously. If the audience takes his side, greater are the chances of the jury doing the same and Atticus is doing his best to achieve the "not guilty" verdict.
