It shows us what God is like and what we are like. Light reveals the true nature of things. If we had been here four hours ago we couldn't see what we look like. It's the same with the light of Jesus. Until we come into the Light we can't see ourselves as we really are - tainted with sin.
People are just like cockroaches.
In the same way, we avoid the light of Christ, because we don't want to admit we are sinful. He can either dodge back into the darkness because he doesn't want to face the dirt he sees on himself, or he can walk into the full light of the forgiveness and fellowship God offers, and to his amazement he finds that the dark stains disappear. The blood of Jesus cleanses from sin. If you have, you no longer walk in the darkness. We have victory over all the powers of darkness.