These are two issues that you either believe or you don't. They endure as articles of faith. They are believed, in large part, simply because the Bible says they are true. Is that enough? Yes, for me, because I find the Bible to be trustworthy, as we've already covered. Is it important to believe them? Yes, to the extent that disbelieving these two doctrines may reduce one's trust in the Bible. I know that sounds circular, but it's not. The circular argument would be "I believe in this doctrine because I believe in the Bible and I believe in the Bible because I believe in this doctrine", but that's not what I'm saying. I'm saying, "I believe in this doctrine because I believe in the Bible, and if I couldn't believe in this doctrine, I couldn't believe in the Bible".
Are the doctrines important in themselves? Yes, again, for in these two phrases are summed up the history-shattering concept of the Incarnation. Other religions have stories of gods taking on human form or humans who achieve godhood, but no other religion contains the idea of this mysterious union of divine and mundane natures. We hear tales of other gods who fathered children with more-or-less willing mortal women but, with very few exceptions, the process described is more-or-less "normal". Not so here, where it seems that the Holy Spirit, a person of the Trinity, simply caused Mary to become pregnant without resort to any physical manifestation. This allowed God to fulfill his promises that the Messiah would be of the house and lineage of King David. It was a mystery to the ancient Jews how a man could be both the Son of David and also the Son of God. These seemingly conflicting requirements were resolved through the Virgin Birth. Jesus is legally descended from David through Mary's husband Joseph and fully the Son of God through the working of the Holy Spirit. In addition, many assert that Jesus is physically descended from David through Mary, helping to solve some problems with disobedience in the kingly line.
It is difficult to take in the magnitude of this unique and unexpected event. As remarkable as it is that anyone could be born of a virgin, it is even more remarkable that God could be born at all. Amazingly, He chose to be born. The Son of God deliberately put off his divine autonomy (not his divine nature) to become completely dependent on the will of the Father. No other religion proposes the existence of a god who would do so much and give up so much for mere mortals.