Is Jesus' conception by the Holy Spirit an important doctrine? Yes, I believe so, for at least three reasons.
First, it calls our attention to Jesus' sinlessness. Numerous verses in the New Testament attest to the sinlessness of Jesus. Allow me to quote just two of them. In 2 Corinthians 5:21 Paul writes of Jesus, "God made him who had no sin to be sin for us, so that in him we might become the righteousness of God." And in Hebrews 4:15 we read, "For we do not have a high priest who is unable to sympathise with our weaknesses, but we have one who has been tempted in every way, just as we are--yet was without sin." According to these verses, and a number of others, Jesus was without sin. And the way in which he was kept from original sin may be traced back to the Virgin Birth.
How was Jesus kept from original sin through the Virgin Birth? Augustine's theory was that original sin is passed down from parents to children through the sexual act itself. Since there was no sex act that led to Jesus' conception therefore he was without original sin. However, Scripture does not spell this out quite as clearly as Augustine might have liked for it to do.
The Catholic Church teaches that Jesus was kept from original sin because his mother Mary was without original sin. The Catholic Church teaches that Mary was immaculately conceived. However, once again, Scripture says nothing of this. More importantly, Scripture suggests that Mary saw herself as in need of a Saviour. In Luke 1:47 she says, "...my spirit rejoices in God my Saviour..."
Others have put forward that original sin is passed through fathers specifically. Since Jesus did not have an earthly father therefore he was without original sin. Once again, it sounds like a broken silent record, but Scripture says nothing of this.
So what does the Scripture say? It says that Jesus was kept from sin by the work of the Holy Spirit. As the angel said to Mary, it was because of the overshadowing of the Holy Spirit that this child would be called "the holy one" of God. (Luke 1:35)
Let us give thanks for the One who was without sin, yet became sin for us that we might become the righteousness of God.