Willy is four years old when he and his brother, Ben, are abandoned by their father. For this reason, Willy's views Ben as both a mentor, a protector, and in a way, as a father figure.
Ben represents to Willy everything that Willy wished he had become: a risk-taker who goes in the jungle at a young age and strikes it rich in the diamond industry; a man who stops at nothing to create new endeavours.
For this reason, Willy has to use Ben's real achievements, as opposed to Willy's invented ones, to illustrate an example of success for Biff and Happy. Willy goes even further: he literally asks Ben to tell Willy whether he is doing things right as far as everything goes. This is the extent to which Willy's self-esteem and self-assurance is as low as it is, compared to Ben's. In other words, Willy needs Ben to assure himself that he is "somebody".
And, yet, as a mentor, Willy uses Ben as his medium between Willy's conscience and sub-conscious while he is contemplating suicide. During one of his fantasy meetings with Ben (who died a few years before the action of the play starts), Willy basically asks for his ultimate approval.
BEN: It’s called a cowardly thing, William.
WILLY: Why? Does it take more guts to stand here the rest of my life ringing up a zero?
BEN [yielding]: That’s a point, William. [He moves, thinking, turns.] And twenty thousand—that is something one can feel with the hand, it is there.
WILLY [now assured, with rising power.]: Oh, Ben, that’s the whole beauty of it! I see it like a diamond, shining in the dark, hard and rough, that I can pick up and touch in my hand. Not like—like an appointment! This would not be another damned-fool appointment, Ben, and it changes all the aspects. Because (BIFF) thinks I’m nothing, see, and so he spites me.
In the end, when Willy does commit suicide, Arthur Miller creates a very Gothic conversation between Ben and Willy. The entire crossover from life to death is interpreted as if Ben has come to take Willy to the adventures on which Willy has, regretfully, never embarked.
Ben, looking at his watch: The boat. We'll be late. He moves slowly off into the darkness.
Willy: [...] Ben! Ben! where do I...? He makes a sudden movement of search. Ben how do I...?
Hence, we see that, from the beginning until the end of his life, Willy has depended on Ben, or on the idea of Ben, to anchor himself as part of the real world. He needs his blessing for most of his adult decisions, and it is Ben who, ultimately, "accompanies" Willy onto eternity.