The plague of flies was a striking display of divine sovereignty. For as Beelzebub, the god of flies, so called, was probably one of the idols of Egypt, nothing could be more admirably suited to show the Lord's displeasure, than by punishing them in the very objects of their own idolatry.
Related Commentaries of Exodus 8:21
Exodus 8:21
21 Else, if thou wilt not let my people go, behold, I will send swarmsd of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are.