Why does Willy tell Charley about Dave Singleman in "Death of a Salesman"? What does Singleman represent to Willy?

I believe it is actually to Howard, his boss, and not to Charley, that Willy relates the story of Dave Singleman.  Singleman was Willy's inspiration when Willy began in the business at a very young age.  Willy recalls that at the age of eighty-four, Singleman could still make a sale with a simple phone call, and that when he died his funeral was attended by hundreds of people.  Singleman was respected and loved by the people in his business, and Willy laments that such recognition and gratitude are no longer possible in the field of sales because the "personality" has been taken out of it.

Willy has just asked his boss for a position where he will not have to travel anymore, and he tells his boss this anecdote because he knows that Howard is not moved by his plea and is about to fire him.  Although Howard is much younger than Willy, he speaks to him condescendingly, calling him "kid".  Willy is trying to tell Howard that even though he is getting old, he can still be useful and successful like Singleman was.  To Willy, Singleman represents a promise fulfilled - he is everything Willy had hoped his own hard years of toil would have made him.

Howard is not impressed by Willy's story, and even Willy has to admit that times have changed.  Whether it is because of the altered climate in the business world or because Willy's personality does not match up to Singleman's, Willy will never be able to duplicate his predecessor's success.