God not only makes promises, He binds Himself to them through a covenant to display His faithfulness and unswerving commitment to what He has said. He does this for our sake so we don't trust our circumstances but trust in God's goodness. This covenant of blessing to the nations through Abram is done by God while Abram is asleep to show that the entire weight of the promise rests solely on God. We can fully trust Him to keep His promise for it does not depend on our performance.