I think that Biff's realization about his own life and the life of his father underscores the theme of the play in a couple of ways. The fact that Biff himself is demonstrating some of the same harmful tendencies as Willy is significant. Biff sees himself believing some of the same elements as his father and embodying the same level of self- hate as his father. It is for this reason that his realization is significant. It shows that Biff will not repeat the same mistakes that Willy has made in his life. Additionally, I think that Biff's realization underscores the idea that the social construction of happiness and success as one intrinsic to wealth and the acquisition of wealth is flawed. Biff recognizes what Willy cannot. It is here where there might actually be some shred of redemption in the process. Biff understands the need to change reality. When he summarizes Willy's own life along with his own, it is a realization that cries out for a different conception of how to be happy:
You were never anything but a hard-working drummer who landed in the ash can like all the rest of them! ... I'm nothing, Pop. Can't you understand that? There's no spite in it any more. I'm just what I am, that's all.
It is here where I think that Biff's realization as to how he can live a different life than his father links to the theme of the drama. Miller would not construct the characterization of men like Willy and Biff without seeing something in the world and recognizing the need to change it into what can or should be as opposed to what is. Biff's realization is the illumination of this idea. When he realizes it, a chance is developed for change. It is here where Biff's realization underscores the theme of the drama.