Be'er Mayim Chaim, Exodus 7:17/באר מים חיים·שמות ז׳:י״ז

3However, the whole essence of their being struck with this plague was for it to thereby be a sign and wonder in the land for the sake of God that there is none like Him. As they were worshipping the Nile, like the statement of the sages, may their memory be blessed (Shemot Rabbah 9:9), [that] their awe was struck, and they all saw that it was [from] the Lord and that there is no savior from His hand. So the idols of Egypt shook before Him. And that is why the section began with this opening, Thus says the Lord, "By this, you shall know that I am the Lord" (Exodus 7:17). For the essence of the giving of this plague was that you should know that I am the Lord; I am He, there is no other, and I will perform signs. So the Lord visited the supernal host above first, and then the kings of the ground, etc., like the statement of the sages, may their memory be blessed (Shemot Rabbah 21:5). And it hints to the life of the husks from which Egypt drew their lifeforce, and upon which they placed their trust.