God stated His reasons for plaguing Egypt

Exodus 6:1 (ESV)

1  But the LORD said to Moses, “Now you shall see what I will do to Pharaoh; for with a strong hand he will send them out, and with a strong hand he will drive them out of his land.”

Exodus 7:1-5 (ESV)

1  And the LORD said to Moses, “See, I have made you like God to Pharaoh, and your brother Aaron shall be your prophet.

2  You shall speak all that I command you, and your brother Aaron shall tell Pharaoh to let the people of Israel go out of his land.

3  But I will harden Pharaoh’s heart, and though I multiply my signs and wonders in the land of Egypt,

4  Pharaoh will not listen to you. Then I will lay my hand on Egypt and bring my hosts, my people the children of Israel, out of the land of Egypt by great acts of judgment.

5  The Egyptians shall know that I am the LORD, when I stretch out my hand against Egypt and bring out the people of Israel from among them.”

Exodus 10:1-2 (ESV)

1  Then the LORD said to Moses, “Go in to Pharaoh, for I have hardened his heart and the heart of his servants, that I may show these signs of mine among them,

2  and that you may tell in the hearing of your son and of your grandson how I have dealt harshly with the Egyptians and what signs I have done among them, that you may know that I am the LORD.”

Vs 1: the reason God hardened the hearts of Pharaoh and his servants (staff) was so that God would have an opportunity to show His signs among them.

Vs 2: the reason God showed His signs among the Egyptians was so Israelite fathers could teach their sons.  As Adam Clarke wrote in his commentary on Ex 10.1, "It was not to crush the poor worm, Pharaoh, that he wrought such mighty wonders, but to convince his enemies, to the end of the world, that no cunning or power can prevail against him; and to show his followers that whosoever trusted in him should never be confounded."

Adam Clarke's Commentary.