This self realisation does add to the tragedy. Through this realisation Biff tries to show Willy that the dream world he is living in isn't real and that he needs to snap out of it and face reality, symbolised by Biff trying to 'make Willy face him' and the truth. However, Willy misinterprets what Biff is saying. When Biff breaks down into tears, Willy believes that Biff is crying for him, not because he has broken down from trying to get his father see the truth. This results in Willy killing himself under the impression that Biff will 'worship' him for it when in actual fact it is the opposite of what he wants, shown by Biff 'rushing down the stairs' to stop him.
Biff's self realisation therefore could be argued to be the cause of the tragedy of the play due to Willy misunderstanding what Biff is trying to say to him.