In Genesis 21:23, this signifies affirmation.

The Lord swearing by himself signifies that divine truth testifies, for He is divine truth itself, and this testifies from itself and means itself. It says 'Jehovah swears' because the children of Israel had a representative church, and so the Lord's conjunction with the church was represented in a covenant similar to two people swearing their trust. This is why swearing was used to seal a covenant or deal. When 'Jehovah swears' it does not mean that He really did swear, but that divine truth testifies or bears testimony to the things asserted.

(Reference: Apocalypse Revealed 474)