As the final plague, the death of the first-born, approaches, Moshe warns Pharaoh that the final plague will kill all the human first-born and "all the first-born of cattle". (Shemos/Exodus 11:5)
The devastating death of the "all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sit upon his throne, even unto the first-born of the maid-servant that is behind the mill" is obvious.
What exactly does the death of all the first-born of cattle add to this, especially since previous plagues, particularly Dever, a plague that killed the Egyptian cattle, already inflicted serious losses on the animals?
When G-d first warns about the death of the first-born, He does so long before in Shemos 4:22-23 even before Moshe has arrived in Egypt.
"And thou shalt say unto Pharaoh: 'Thus saith the LORD: Israel is My son, My first-born. And I have said unto thee: Let My son go, that he may serve Me; and thou hast refused to let him go. Behold, I will slay thy son, thy first-born.'"
There's no mention of animals. Why then do the first-born of animals suddenly make an appearance?
As the final plagues approach, Pharaoh continues his bargaining strategy. At the beginning of Parshas Bo, when threatened with a plague of locusts, he offers to let the Jewish men, but not the children, go to worship G-d. (Shemos 10).
After the plague of darkness, Pharaoh agrees to let all the Jews go. "Go, serve the LORD; only let your flocks and your herds remain; let your little ones also go with you", only for Moshe to retort, "You too will also give us hand sacrifices and burnt-offerings, that we may sacrifice unto the LORD our God. Our cattle also shall go with us; there shall not a hoof be left behind; for from it we must we take to serve the LORD our God; and we know not with what we must serve the LORD, until we come there."
It's this attempt to hold on to the animals that triggers the final plague, the death of the first-born of both humans and animals.
The final plague, the death of the first-born, encompasses the final two bargaining elements that Pharaoh had sought to impose, the children and the animals.
These two elements, the children and the sacrificial animals, are also the essence of both how the Jews were oppressed in Egypt... and of Pesach/Passover.
The Jews arrive in Egypt to act as Pharaoh's herdsmen. When a future Pharaoh decides to wipe them out he begins killing their children.
Parsha Bo begins with the birth of Judaism as G-d tells Moshe, "Go to Pharaoh; for I have hardened his heart, and the heart of his servants, that I might show these My signs in the midst of them; so that you tell in the ears of thy son, and of thy son's son, what I have wrought upon Egypt, and My signs which I have done among them; that ye may know that I am the LORD.'"
G-d is no longer simply intervening in the matter of the enslaved Jews because he has a covenant with their ancestors or out of a matter of justice, but to build a deeper ongoing relationship with the Jews.
The events transpiring in Egypt are meant to build a larger legacy and to be recalled by their children as an annual event giving rise to the first Jewish holiday, Passover, and the first commandments of Judaism.
The transmission of a relationship with G-d through the children is the essence of Judaism. It is certainly the essence of Passover which to this day recreates the story of Egypt through the recital of the story in the Haggadah at the Seder.
The other core element of Passover, not practiced today due to the lack of the Holy Temple, is the Passover sacrifice.
What's the significance of animal sacrifice, an act that strikes many as barbaric today? Animal husbandry, the core of economic life in the ancient world, represented human labor. The act of animal sacrifice goes back to Kayin and Hevel, the first sons of man, of Adam and Chava,
Hevel or Abel brings "of the firstlings of his flock" to G-d. (Bereishis/Genesis 4:4) as an acknowledgement that the best, the first, of his labor belongs to G-d who enables man to succeed.
Likewise, the birth of a son, Kayin, leads Chava/Eve to proclaim, "I have acquired a man with the help of the LORD."
Children and our economic achievements are the result of a partnership with G-d. By raising children in the way of G-d and by bringing sacrifices, Jews acknowledged that everything we have comes from G-d.
Until the very end, Pharaoh does everything he can to obstruct this basis for religion, fighting against the Jewish determination to educate our children in the way of G-d, as many tyrants would go on to do throughout history, and then to obstruct the animal sacrifices that acknowledge that what we have comes from G-d.
Pharaoh did everything he could to stop Pesach from happening. The holiday is a testament to his failure and the inevitable failure of those tyrants like him who will ultimately fall to the will of G-d.