Sforno on Exodus 10:1/ספורנו על שמות י׳:א׳

1כי אני הכבדתי, even though Moses had said; “I know that you do not yet relate with awe and reverence to G’d,” (9,30) he thought that even if he does not humble himself to G’d because of G’d’s greatness, at least he should do what G’d says seeing that he has no choice, and can no longer withstand the cumulative effect of the plagues. He had arrived at this conclusion when he noted the words ה' הצדיק, “the Lord is the Just One.” However, when he found out that all these pious words notwithstanding Pharaoh continued to oppose G’d’s will in spite of the fact that he found it impossible to cope with the plagues, Moses had come to the conclusion that warning Pharaoh of an impending plague was an exercise in futility. This is why G’d had to tell him at this stage that already during the sixth plague (9,12) He had stiffened Pharaoh’s heart so that ordinary rules of psychology could no longer be applied to this man. The purpose was to enable G’d to demonstrate more miracles so that maybe some Egyptians would be moved by what they experienced to become penitents. If so, the Israelites in the future would be able to tell their children of the greatness of G’d’s miracles. This in turn would convince mankind that G’d loved His creatures and was very patient with them, giving them opportunities to mend their ways. This is why the warning to Pharaoh was in place although it would prove ineffectual. G’d’s plan was

2למען שיתי אותותי אלה אלה בקרבו, so that I can demonstrate these miracles of Mine in its midst, so that the people realise My greatness and return in contriteness.