In Death of a Salesman, Arthur Miller provides powerful themes which are supported by the use of flashbacks.
Willy Loman is a man who lives in the past. His sense of success is based upon achievements he had when he first started out as a salesman. However, he has failed to change with the times, and is stuck within the confines of a world that no longer exists outside of his own mind.
Four major themes are appearance vs. reality, the individual vs. society, the individual vs. himself, and the perception of the American Dream.
Loman's insistence upon living in the past is directly related to all of the major themes. In terms of appearance vs. reality, Willy believes that his son Biff can do anything he wants. He does not see who Biff has become, but is tied to his vision of the son who was a football star (flashback), who was to receive a football scholarship—but loses by failing math in his senior year. Willy doesn't even recall the fact that Biff never graduated high school. Willy finds fault with the choices Biff makes in trying to get by, torn because as in the past, he believes every door is open to Biff if he will only try.
Another of Willy's struggles deals with the theme of the individual vs. society. Willy's connection to the past keeps him from seeing that he is a dinosaur, that sales have fallen into the hands of the younger generation who do things differently, and so get the business his company wants...in this, he finally loses his job because his boss can not carry Willie's "dead weight."
Individual vs. self is seen in Willy's inability to understand what is going on around him. Through flashbacks, he hears the voice of his brother (who is dead--appearance vs. reality) and struggles with what he believes should be true of his life, and what has not happened in his desire to be the best salesman ever. Now he struggles financially, has been involved with another woman, has alienated Biff in the process, and then gets fired. He struggles with how to make things right, and has often contemplated suicide when things have gotten really bad.
The American Dream is what started Willy on his journey to being a great salesman: he believes that in the US, all things are possible. He is drawn into the past when he met a successful salesman whose lifestyle motivated Willy to go into sales. However, as is the case today, making more money was desired rather than just making some money. And while the dream was attainable when Willy was young, he is now in his sixties, and he cannot compete with younger salesmen: the American Dream is no longer his.
The flashbacks include reliving the time when his brother Ben encouraged him to go to Africa with him to make enormous amounts of money, which he never does, and regrets (individual vs. self). Flashbacks show Willy's son Biff with his cousin Bernie who is always pushing Biff to study for school and issuing dire warnings of failure if he doesn't do his work.
As the story progresses, Willy finds his reality getting mixed up with the past through the intermingling of the present day and flashbacks. As this continues, Willie begins to fail, losing his grip on reality, facing life's disappointments, and the missed opportunities that have shaped his life and that of his family. The flashbacks serve to show him how he has failed in life.
Ultimately, Willie takes his own life.