Willy is lost in his mind, thinking of an earlier time when Biff and Happy were young and the prospects for the "greatness" of his sons was still possible. He speaks through the wall as if the younger Biff and Happy were really on the other side. He *must* pretend that his sons are out of sight, so as not to disrupt his fantasy. The sight of the grown boys would dissolve his illusion.
The stage notes here read: (Willy is gradually addressing -- physically -- a point offstage, speaking through the wall of the kitchen, and his voice has been rising in volume to that of a normal conversation.)
It is important direction for understanding the Willy's character. He starts off just rather talking to himself, but the longer he goes on with the narrative, the more he convinces himself that it is true, and his voice rises correspondingly.