Ben is Willy's older brother, a figure he looks up to. A figure he is always trying desperately to impress, even though Ben has passed away. In a way, Ben haunts his younger brother with his failure to be successful and achieve the American Dream. Ben is said to have walked into the African jungle at the age of seventeen, and when he walked out at twenty-one, he was rich "by God." It is assumed that he got into the business of diamond mining to earn his riches. Why Ben never deigned to share with his brother Willy is uncertain.
Whenever Ben shows up throughout the play, during one of Willy's numerous and periodic flashbacks, he causes Willy to become over-exuberant and a bit pathetic. Willy seems to simper in the presence of his brother, his idol. It is rather like the way Happy looks up to Willy as his father. It is reasonable to infer that Ben acted as Willy's father figure in their father's absence. Ben serves as a reminder to Willy of what he could have been, and the things he could have accomplished. He stands as a real-life example of one of the few lucky people ever to attain the American Dream. To Willy, Ben holds the secret to success, so whenever he appears, Willy immediately focuses solely on him, asking him questions and consulting him about his life. Although, to be honest, the way Ben earned his wealth is questionable. He was originally supposed to travel to Alaska and ended up in the African jungle... He makes like he didn't know he'd traveled to an entirely separate continent, which is pretty unbelievable. Ben even mentions the fact that he's built an empire and never kept any books. People who don't keep track of their money usually tend to lose it. It is not clear whether he is even telling the truth about his rags to riches story, given that he would never go into to detail and only kept repeating "When I was seventeen I walked into the jungle, and when I was twenty-one I walked out. And by God I was rich." That is the extent of his story, the only piece of information he really gave anyone.
In all honesty, I would rather see a man who works hard for his money, see him get to experience a struggle, because that is when respect is earned. Easy money builds no character, and it is clear that that is something Ben does indeed lack.