The requiem allows for a thematic closing statement in the play and also softens the ending, offering a continuation of "life" in the story after Willy's suicide.
Willy dies at the close of the last act, but the play goes on. This provides both a figurative and a literal continuation of life for the Loman family and helps to soften the brutality of Willy's death.
Also, the themes of the play dealing with the American Dream, illusion/self-deception, and the idea of success are given final voice in the requiem.
Linda speaks for the dignity of the average man. Biff speaks for the tragic lapse in judgement that characterized his father and which was rooted in a false dream of greatness. Happy speaks for the quality of the illusion that was instilled in him by his father and his father's country. The American Dream still holds sway with Happy, despite the fact that it essentially ruined his father.
In the end, all that Willy achieved was a quiet family life. Linda says that she has made the last payment on the house. According to Linda and Biff, that should have been enough for "success". Yet, Willy took this life for granted and wanted more. He did not know who he was, really, or what he actually had, as Biff says:
Willy "never knew who he was" and that he "had the wrong dreams."
Happy, for his part, defends Willy.
"He had a good dream. It’s the only dream you can have – to come out number-one man.”
The conflict between these views of Willy Loman articulates the conflict of and within Willy's character.