I think that by the end of the play, Biff does achieve a certain level of self realisation. Throughout the play, he has been striving to achieve "the american dream", an idea Willy put in Biff's head, "you blew me so full of hot air". Biff says "why am I trying to become what I don't want to be?" showing he has finally realised that he doesn't want to work in an office, "what am I doing in an office", and that he wants to be outside with "the things that I love in this world". It also becomes apparent that Biff realises that it is Willy who has made Biff chase the things he doesn't want. The line "Why can't I say that, Willy?" hints that Biff thinks it is Willy's fault he has wasted his time and he is blaming him for it. He says "I'm nothing...that's just what I am, that's all" showing he has realised and accepted who he is.
I think Biff's self realisation does add to the tragedy because his realisation causes another in Willy, albeit a misguided one. Biff's realisation serves as the trigger for Willy to act and ultimately kill himself. Willy is utterly convinced he is making the best choice for Biff, "it's the best thing" when he has just misinterpreted what Biff was telling him. Biff's realisation also creates a peak in the tension. All of the things that Biff never said to Willy coming spilling out in rapid succession, creating lots of pace, which adds to the tension.
