Orthodox icons of Jesus emphasize humanity and divinity in one person. Notice how the face is asymmetrical (like most human faces). But in Jesus' case, he is purposely portrayed this way by the iconographer to emphasize divinity and humanity. Notice how Jesus' left eye is slightly larger than his right eye and his left eyebrow slightly higher than his right. That is because the left side of his face is intended to depict his divinity and the right side his humanity.
A second reason why Jesus' conception by the Holy Spirit is important is because it calls attention to Jesus' divinity. The two things are not synonymous. Just because Jesus was born of a virgin that does not necessarily make him divine. And I suppose that Jesus could have been divine without being born of a virgin. But if Matthew and Luke are correct, then this is the manner in which God chose to take on our human flesh. There is a certain appropriateness to the divine Son of God being born in this manner. It seems only right that Jesus was born not by human initiative but by divine initiative.
On the other hand, a third reason why Jesus' conception by the Holy Spirit is important is because it calls attention to the fact that Jesus started a new line of humanity. In 1 Corinthians 15:45 Paul wrote, "So it is written, 'The first man Adam became a living being'; the last Adam, a life-giving spirit." Both in 1 Corinthians and in Romans 5, Paul referred to Jesus as a kind of second Adam, the first in a new line of humanity. The Virgin Birth calls attention to this: that something new and completely different is happening in Jesus.
Yes, Jesus had a human line of ancestry through Mary going all the way back to Adam (according to Luke's genealogy). However, in another way, Jesus began an entirely new line. Jesus was, to use theological terminology, a new covenant head. Just as Adam by his disobedience brought sin and death and judgment to all, so Jesus through his obedience brought grace to all who believe in him. The Virgin Birth draws our gaze to look upon this truth, that here we have an unusual person, one who was without sin, one who was fully divine and fully human at the same time, and therefore one who could act as a covenant head to earn eternal life for all who believe.
Karl Barth put it this way: "According to Scripture and creed, Jesus Christ is not the second or new Adam because He was born of the Virgin. His being the second or new Adam is indicated--'That ye may know...'--by His being born of the Virgin."
This part of The Apostles' Creed witnesses against an ancient heresy called adoptionism. Adoptionism was briefly popular in the period of the early church. The early church was wrestling with the question of when Jesus became the Son of God. The answer of adoptionism was that Jesus was merely adopted by the Father at the point of his baptism when the Father said, "This is my Son, whom I love." However, Matthew and Luke, by the doctrine of the Virgin Birth, suggest that Jesus was fully God from the point of his conception, not to mention the fact that he was the Son of God from all eternity, even before his incarnation.
Thus, this line in the creed ("who was conceived by the Holy Spirit") is important for at least three reasons. With one stroke it calls attention to Jesus' sinlessness, his divinity, and his role as a new, human, covenant head.