God's Purpose in Pharaoh's Hardening

Exodus 11:9-10

And Jehovah saith unto Moses, `Pharaoh doth not hearken unto you, so as to multiply My wonders in the land of Egypt;'

and Moses and Aaron have done all these wonders before Pharaoh, and Jehovah strengtheneth Pharaoh's heart, and he hath not sent the sons of Israel out of his land.