The virgin conception teaches us that the nature of Jesus is divine. He was conceived by the Holy Spirit. His concepcion was a miraculos event, engineered by the Supernatural God. He wasn't a generic copy but the 'real thing'...God in the flesh. Eugene Peterson, in The Message, says that we could paraphrase John 1:14: "The Word (God) became flesh and blood, and moved into the neigborhood."
The teaching of the virgin birth is central to the Christian message. Only a divinely orchestrated Messiah could have fulfilled all of the Old Testament prophecies from 2d Samuel 7:16 through Daniel 7:13-14 and Micah 5:2 to Mary, without violating anyone's freewill. Therefore, to say "Jesus is Lord" is to say "Jesus is Divine" That's what got the early Christians in trouble with the Roman Empire at the end of the First Century. They refused to say "Ceasar is Lord (divine)" because they believed that their discipleship to and belief in Jesus required trusting that he and no other was Divine.