In Arthur Miller's play Death of a Salesman Willy is in the middle of one of his many hallucinations while he is compulsively planting seeds in his garden. This particular hallucination involves Willy's long dead brother, Ben, who is Willy's older brother by about 15 years.
Ben is a very influential person in Willy's life. Not only is he successful and rich, but he is also a father figure to Willy, and even to Willy's sons, until his death. After all, he does help raise Willy after their father abandons them.
Hence, it is significant that Ben would be the person to whom Willy confesses his idea of cashing in his life insurance by killing himself.
The response that Ben gives Willy is obviously a figment of Willy's own thoughts and imagination, which is why the message is that Biff would never forgive Willy if he carries on with that plan.
Ben also says that the insurance company may not even grant Willy his money, since it involves a suicide. In all, this is demonstrative of how Willy's thoughts would flow and how the idea of suicide is ever-present in his mind.