Ben Loman affects Willy tremendously because his character is a foil of Willy's.
Older than Willy Loman, Ben was Willy's support system when their father left the household. However, the first difference we encounter in the brothers, aside from their age, is that Ben is courageous, and set out to look for their father in Alaska. Contrastingly, Willy is left defenseless, and alone.
Secondly, Ben was daring and willing to initiate and take on challenges. One of them left him a very rich man when he decided to take the risk of entering the African mining business. In contrast, Willy never became successful and led his life trying to chase after the successes and dreams of people who led their own goals,namely, Dave Singleman: The man whose success inspired Willy to be a salesman for the sake of becoming rich quick.
Third, Ben's success rubs on Willy's ego at all times. Certainly Willy loves his brother, but it is impossible to maintain a healthy brotherly relationship when Ben continuously reminded Willy about his lack of success.
In all, Ben shares the same ambition and desire for a better life as Willy. However, as often happens in life, success is achieved by the survival of the fittest. In this case, Ben was the fittest man and became successful. As a result, Willy had to endure the act of basking on his brother's glory or accept his own failures.