The angel Gabriel was sent to Mary six months after he had been to the parents of John the Baptist to announce to her that she had found favour with the Lord God and would be the mother of Jesus. The immaculate conception is one of the hallmarks of the Christian faith and teachings as it was by this miracle the the Lord came to the earth.
Mary naturally was troubled by the visitation of the angel Gabriel, but was put at ease when he explained what was to happen. She made a valid point in that she had no husband at that time but was only betrothed to Joseph. We see later that she must have come under great pressure for Joseph had resolved to separate from her after she became pregnant, believing that she had been wanton and unfaithful. That would be the normal way that people would have thought in such circumstances. But when Joseph was made aware in a dream that she was innocent and that she had conceived by the direct intervention of God, he changed his mind.
It was necessary that the Lord be born of the holy spirit and not conceived as a normal man. He had to be the Son of God so that he could be born without sin and live his life free from the sin factor in order to become the perfect sacrifice for sin. This is why the immaculate conception was so important. Without it he would have had the same weaknesses as the rest of mankind and be unable to bring forth the New Covenant that gives us freedom from sin today.
When the angel said Jesus would be given the throne of his father David, it is clear that Mary was of the lineage of David, as too was Joseph. Jesus was born into the tribe of Judah which was the tribe of the kings. And when the angel said he would rule over Jacob forever, he is speaking of the spiritual kingdom of Israel. That is, those who are the sons and daughters of Abraham through faith rather than bodily descent.
We are Abraham’s children too through faith, regardless of our physical lineage. It does not matter what our history and lineage is, when we come to the Lord in faith and enter into his death through baptism, we become one with him and through faith we are children of Abraham and children of God.