Either an eavesdropper or witness to something. Possibly there to witness what was happening in case people started asking questions.
Looks to be either a 16th century or 17th century piece. Typically during this era at least in terms of Biblical paintings, they add somethings to better enhance the story of a painting. Since art after all tells more than words and is interpreted differently.
I think maybe the artist ) felt it was too...I dunno farfetched that there weren't other onlookers. Unless it's a representation of Esau himself? I think the older man is Issac judging by the other paintings I looked up.