Jesus being born of a virgin was far more than just a sign to the world, although it certainly was that.
Joseph and Mary both had a claim to the throne of David. However, Joseph's claim was invalid as he is a descendant of Jaconiah, who was cursed and whose line could never ascend to the throne. So had Jesus been the biological son of Joseph, Jesus would not have a valid claim to sit on David's throne.
As Mary, with no brothers, could only take land and inherit her birthright if she married within her tribe, she faithfully married someone within the tribe of Judah, her tribe, and the tribe that held the scepter.
So Jesus is born of two people of the House of David, which is why they had to report to Bethlehem, the City of David, for the census. But Jesus' claim to the throne of David is pure, as he is not biologically related to Joseph.
Further, as we all inherit the sins of our father, Jesus' father, God the Father, had no sin to inherit, and thus Jesus was born sinless.
Again, the mother earth cult in the 1850's declared that Mary too was born sinless, but this is an obvious attempt to equate her with the Son of God, as is calling Mary the Mother of God.
Mary is the mother of Jesus, and the mother of the Messiah, and the mother of the Lord, but never in the bible, or in reality, is she properly called the mother of God. Calling Mary the mother of God is a claim to divinity, just as Jesus calling himself the Son of God caused the Jews to pick up stones with which to kill him for blasphemy.
After all, kind begets kind, always, and God can only beget God.