There was the true Light which, coming into the world, enlightens every man.
-John 1:9 (NASB)
If you've ever been in a cave, you know how a tiny bit of light makes an enormous difference in the darkness. Here, the evangelist compares Jesus to light. He makes the remarkable claim that, through His coming into the world, Jesus brings light to each person.
The unspoken assumption here is that the world was enveloped in darkness before Christ came into it. Perhaps it was not an utter darkness, but it was dark enough that there was room for each of us to be "enlightened."
In the light that Christ exudes, we begin to see the truth about the Life, the Universe, and Everything. We see God's love, our own self-focused disobedience, and the way that our behavior has corrupted God's good world. We see human pain, loneliness, and need, and we see the solution that God has provided for these problems.
When the light shines in a dark place, we suddenly see everything: The beauty that was hidden only moments before, and the ugliness that lurked out of sight. It is necessary, this enlightenment. Without it, the beauty would go unappreciated, and the ugliness would linger on, festering and growing ever uglier.
We need the light if we care about knowing the truth.