The sun god of Egypt was Re (Ra) and was represented by Pharaoh himself. He was held in high esteem as he represented the sun. The darkness showed how powerless even the mightiest of their gods was compared to the God of the Israelites. Three days of darkness would have disrupted the ability to function in much the same way that the plagues of animals had before. Again we see that God’s people had light where they lived. They were set apart in more substantial fashion with each plague.