‘And Laban said, “This heap is witness between me and you this day.” That is why its name was called Galeed and Mizpah (place of watching), for he said, “Yahweh watch (zapah) between me and you when we are hidden (i.e. out of sight) one from another.”
Laban, as head of the confederacy of which Jacob had been a part, takes the leading role in declaring the significance of what is happening. The heap is mentioned because that represents Laban's part in the covenant. It is their witness to Jacob. Thus it is now given another name, ‘place of watching', for it not just a witness to the covenant but the place from which God will observe for the fulfilment of the covenant. He will, as it were, stand on that border and guarantee the fulfilment of the covenant on both sides.
“Yahweh watch between me and you.” Laban cites Jacob's God for it is Yahweh Who will watch for Jacob and see to the observance of the stipulations that follow.