The main reason for Willy's older brother Ben being in the play is that he gives Willy someone to confide in. Stage plays rely heavily on dialogue. If Willy didn't have Ben to talk to about his private thoughts, there would be no way to convey such information to the audience--except for resorting to soliloquies, as in Shakespeare's Hamlet. Soliloquies seem unnatural because they are unnatural. They would seem especially out of place in drama that is naturalistic like this one. Yet somehow it does not seem unnatural for Willy to be holding conversations with Ben when we are well aware that Ben is only present in Willy's mind. Evidently this is because this is something we all do--or most of us, anyway. We imagine ourselves having conversations with "important others" in our lives, often trying to justify or rationalize what we have done, or what we intend to do, or what we failed to do. These important others will usually include fathers, mothers, former friends, and former lovers. They also come to us in our dreams. Whoever they are, they are people who made their mark on us. They still live inside our minds, even though many of them may be dead.
A good example of Willy's relationship with Ben is found towards the end of the play, when Willy seeks his brother's approval of his plan to commit suicide in order to let his son Biff collect the $20,000 premium. Willy will fake it to look like an accident because the policy pays double indemnity.
WILLY: Can you imagine that magnificence with twenty thousand dollars in his pocket?....Imagine? When the mail comes he'll be ahead of Bernard again!
BEN: A perfect proposition all around.
WILLY: Did you see how he cried to me? Oh, if I could kiss him, Ben!
BEN: Time, William, time!
WILLY: Oh, Ben, I always knew one way or another we were gonna make it, Biff and I!