Willy's hallucinations feature his brother Ben. Ben stands as the image and definition of success for Willy.
Ben is a bold and adventurous man with great charisma who uses his talents to make a fortune. Willy sees greatness in his brother and attempts to find that greatness in himself.
Importantly, Ben appears only in the form of a hallucination. Willy's image of success is literally a delusion in this regard. Despite the illusory nature of Willy's dream for himself and for Biff, Willy clings tightly to it, choosing to die instead of admit defeat.
The hallucinations serve to provide the reader with a source for Willy's dream as well as a demonstration of Willy's mental state. Though his demented visions are benign, they nonetheless clearly suggest Willy's state as one of illness, fantasy, and rather profound disappointment.