Or HaChaim on Exodus 10:2/אור החיים על שמות י׳:ב׳

1 ולמען תספר, "and in order for you to tell, etc.," Why did G'd have to provide this additional reason to justify the plague of locusts? Besides, what is the meaning of the words אשר התעללתי במצרים? Why did G'd add: "and My miracles," as if this was something distinct from אשר התעללתי? Why did G'd add: וידעתם כי אני השם?

2I believe that first and foremost G'd wanted to make it plain that it was not His purpose to bring on the plague as an act of revenge on Pharaoh. Rather, the miracle was designed to strengthen Israel's faith in G'd so that they would remember these miracles forever. This could be achieved only if G'd not only performed the miracle without hurting any Israelite by it but that Pharaoh himself would be so impressed by the miracle that it would also impress G'd's power on the minds of the Israelites in an unforgettable manner. The psychology of the aforesaid is demonstrated in the halachah of the בן פקועה in Chulin 75. This concerns a fetus found alive [if the pregnancy was incomplete, or dead if the pregnancy was complete. Ed.] inside an animal after the mother-animal has been ritually slaughtered. The question is if the ritual slaughter of the mother-animal made the fetus fit for consumption by Jews without further ado. The Talmud discusses whether such an animal may be eaten without it being slaughtered seeing that the onlooker may suspect the person eating it of committing a sin; it concludes that when this animal has been different in at least two exceptional ways such as that it does not have a cloven hoof, everyone will remember what its origin has been and one will not suspect someone eating it without having performed ritual slaughter as having committed a sin. Although G'd had demonstrated miracles which clearly proved His power, this would not have sufficed to make a lasting impression on the Israelites unless He had at the same time proven that He distinguished miraculously between the guilty and the innocent, i.e. between the Israelites and the Egyptians and that the miracles had hurt only the guilty parties. When G'd said: "in order that you will tell your children and your children's children, etc." He meant that this would be assured only if He performed the miracles in such a way that only the Egyptians were seen to be hurt by them. The first impression would be created by אשר התעללתי במצרים, "How I wrought the plagues on Egypt;" the second and more lasting impression would occur when the Israelites realised את אותותי אשר שמתי בם, that what occurred was an outstanding miracle even if the Egyptians had not been a factor at all. These combined experiences would prompt the Israelites to tell their children about what they had seen. The purpose of telling their children was "so that you will know beyond doubt that I am the Eternal G'd."