What is a requiem? What is the purpose of this final act? To what extent is it successful?

A Requiem is a Mass or speech or even music celebrated for the dead

The requiem is the last act of Miller's play where the sad truth of Willy Loman's existence is revealed to the audience and the Loman family. The requiem serves as a place where Miller paints a picture of Willy's death as an ironic end to his tragic life. Check out a pretty good essay on this at my link below.