The importance of the requiem in the play Death of a Salesman, by Arthur Miller, is that it reinstates the main theme of the story, and explains (and forgives) the character of Willi Loman.
In this part of the play, we find Bernard, Charley, Biff, Happy and Linda standing together as Willy's only mourners. As usually happens in funerals, or wakes, people tend to look back and analyze the person who just passed. In this particular conversation Biff states his opinion that Willy just had the wrong dreams all along.
However, in defense of Willy, Charley counter argued Biff by stating that all the things Willy did were part of what makes a true salesman: Salesmen need to dream, envision, hope, and maintain a "go get it" attitude. He also stated how the salesman's life depends entirely of the ability to hope. Maybe Willy was more of a survivor than a wretch. He really never gave up being a salesman.
Therefore, the requiem helps us look back in the plot and conclude whether Willy is a hero, an antihero, or a victim. We can conclude, from what Charley stated, that Willy may have chased the wrong dream, but he never gave it up. He made a wrong decision but went with it until the end. This allows the reader to take a lot of the negativity of Willy's character away and, perhaps, it also prompts the audience to "forgive" Willy.