Proper answer , i have not a clue, although the idea to thread with a weaker part in this case a thinner twine is to make a weak point so that will give first, often used in a sand anchor. If the anchor gets lodged and fast in a rock the twin breaks which puts the anchor at a different angle to pull up often enabling it free of the rock.
Though in this case I think it was put there to hand a for sale sign on. Seems the sign wore away before the boat.