This forewarning God gives, partly, to initiate the severity of the judgment; partly, that a considerable number of horses might be reserved for Pharaoh's expedition, Exo 14; partly, to show the justice of God in punishing so wicked and obstinate people, as would take no warning neither from God's words, nor from his former works; and partly, to make a difference between the penitent and the incorrigible Egyptians.
Related Commentaries of Exodus 9:19
Exodus 9:19
19 Send therefore now, and gather thy cattle, and all that thou hast in the field; for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die.