What's the purpose of the plagues? God wants Israel to know that He's the Lord, their deliverer; that He's going to redeem them with an outstretched arm, and He tells them ahead of time so that they'd know it was Him. But he also wants Egypt and Pharaoh to know that He is Lord as well. In that, the plagues serve as judgements for 400 years of brutal infanticide and slavery, for which Pharaoh refuses to repent.
