Ben is Willy's ideal of "success incarnate," the self-made man. Also, as Willy's brother, he serves well to make Willy feel not just a bit inferior. When he first appears in Act 1, Willy, exhausted but unable to sleep, is playing cards with his neighbor, Charlie. Willy does his best to talk, at the same time, to Ben, who is but a memory, and Charlie, who is right there across the kitchen table from him. More and more lost in the past and with little real hope for a better future, Willy is having trouble holding it together in the present.
Fed up trying to relate to Willy, Charlie quits the card game and leaves Willy. Willy is then alone in the kitchen and can speak freely to the brother of his past:
WILLY (turning to Ben): Business is bad, it’s murderous. But not for me, of course.
BEN: I’ll stop by on my way back to Africa.
WILLY (longingly): Can’t you stay a few days? You’re just what I need, Ben, because I — I have a fine position here, but I — well, Dad left when I was such a baby and I never had a chance to talk to him and I still feel — kind of temporary about myself.
With no role model, no stable father-figure in Willy's life, he has probably always been unsure of himself, of his roots and his own true nature. All he has now is the vague memory of a father who abandoned him and his family, and his brother who also deserted him.
No wonder he feels more than a bit unsure of himself, ungrounded, incomplete... temporary.
Ben is Willy's ideal image of success. Ben has been who he has always looked up to and when Willy's life was falling apart and his mental state was already in disarray, he remembered his brother Ben and believes Ben is helping him, while in reality this is simply his illusion. He holds on the past when he is trying to hope for a better future, and this is why he keeps on seeing and talking to Ben.