What is the importance of Dave Singleman in Death of a Salesman?

Dave Singleman was part of the reason that Willy did not
follow his brother out to the lucrative gold fields of Alaska. Dave Singleman was a
salesman in his eighties, well respected and successful. This was a very attractive
future to Willy, and he was mesmerised with the promise of comfort and recognition that
Dave Singleman’s career exemplified –

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 WILLY:…I realised that selling was the greatest
career a man could want. ’Cause what could be more satisfying than to be able to go, at
the age of eighty-four, into twenty or thirty different cities, and pick up a phone, and
be remembered and loved and helped by so many different
people?

Willy was
impressed also with the recognition Dave Singleman received after death, as ‘hundreds’
of buyers and salesman from all over the country attended his
funeral.

There is also poignancy in that the title of the
play is taken from the passing of Dave Singleman, and shows how Willy would have liked
to have passed-

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 WILLY: …and by the way he died the death of a
salesman, in his green velvet slippers in the smoker of the New York, New Haven and
Hartford...