Why do you think Ben reappeared at the moment of Willy's final decision?

Willy Loman's late brother Ben makes regular appearances in his fevered imagination in the form of an apparition. He represented everything that Willy can never be—rich, successful, well-liked. In his life, he was the epitome of the American Dream by which Willy tries so hard to live, but which will eventually destroy him.
When Willy finally makes the decision to end his own life, Ben appears once more. This time he tells him that his suicide will be like a diamond, that is tangible, something you can reach out and touch. Ben made his fortune in the diamond trade, and in Willy's apparitions tells him that the "The jungle is dark, but full of diamonds." In other words, however hard life appears to be, there are always opportunities out there for the right man.
Willy comes to see his imminent suicide as the grasping of just such an opportunity; not for himself, but for his family. With the money they'll collect from the insurance policy they'll be able to take their own shot at the American Dream. In that way, Willy's death will be his only real success; the only way he could ever hope to emulate his late brother.