Matthew Poole and continuators on Genesis 21:27

27 And Abraham took sheep and oxen, and gave them unto Abimelech : and both of them made a covenant.

Partly as an acknowledgment to him for his former favour and Friendship; partly as an assurance of his sincere Friendship both present and for the future, of his aquiescence in his answer about the Well; and partly for Sacrifice, and for the usual Rite in making Covenants, which was that the persons Covenanting might pass through the parts of the slain Beasts. See Gen. 15. 18.