Ben is both Willy's symbol of success ("That man was a genius, that man was success incarnate!") and also the only person in Willy's family left to talk to in his imagination. In many ways, Willy looks up to Ben for approval and advice, just as he would a father.
And in the end, it is to Ben that Willy goes to ask advice about his contemplated suicide. He comes to Ben with the idea as if it were a business deal. Ben is everything Willy never was: he's strong, resourceful, independent, self-assured, self-made, self-important and rich.
Ben thinks over the deal, the suicide that will bring Biff money at last, and he agrees with Willy, after some reservations, that it's a good idea. Propetically he says to Willy, "The jungle is dark but full of diamonds..."
His doom is set in a final conversation with his dead, imaginary brother, and Willy drives off one last time.