Physical labour is something that is incredibly important in the way that the characters of Biff and Willy are described to us. It becomes very clear, as the play develops, that Willy's dream is "all wrong" because he would have been much happier working as a carpenter or doing something with his hands rather than trying to force himself to become a salesman where he only enoyed mixed success at best. In the same way, Biff, tainted by his father's dream, seems to ignore what he knows, at least in some level, is his calling. Note what he says to Happy when they are talking about going West together:
Sure, maybe we could buy a ranch. Raise cattle, use our muscles. Men built like we are should be working out in the open.
Biff recognises that the city does not suit them, and he also explicitly refers to the way in which Willy was "never happier" when he was working on the house. Note how in the Requiem he refers to the way in which there was more of his father in the stoop he built than in any number of sales he made. Physical labour is part of the tragedy in this play, as it is shown to be what Willy and his sons truly excel at. Their realisation of this and what would truly make them happy is blinded by the unattainable nature of the American Dream, however.