As with every requiem, the purpose of it is to honor the dead by remembering their life. It is, literally, the last first that the now deceased will be honored by the living, and it is often expected that only the good memories come up and happy thoughts are brought to the sad moment.
Unfortunately for Willy, we see that his disparate life has resulted in an equally disparate death; instead of the grand funeral that Willy once dreamed of having, the one that would be as big as Dave Singleman's, he had nobody show up with the exception of Charley, and of Willy's immediate family.
LINDA: Why didn’t anyone come?
CHARLEY: It was a very nice funeral.
LINDA: But where were all the people he knew? Maybe they blame him.
In a dramatic contrast to the funeral that celebrates a happy, or at least fulfilled, life, Willy's was mainly another family quarrel where Biff downplayed all of his father's life. As a result, Charley has to explain to both Happy and Biff that the life of a salesman is thankless; that all a man has is his dreams when dreams are the basis of the job that you do. In other words, it is Charley who honors Willy's life as best as he knows how. That is the idea behind adding the requiem at the end. It is Willy's final chance at redemption even if it is merely for the sake of his funeral.