"The Mystery of the Annunciation is the Mystery of Grace" by Joseph Ratzinger
The mystery of the annunciation to Mary is not just a mystery of silence. It is above and beyond all that a mystery of grace.
We feel compelled to ask ourselves: Why did Christ really want to be born of a virgin? It was certainly possible for him to have been born of a normal marriage. That would not have affected his divine Sonship, which was not dependent on his virgin birth and could equally well have been combined with another kind of birth. There is no question here of a downgrading of marriage or of the marriage relationship; nor is it a question of better safeguarding the divine Sonship. Why then?
We find the answer when we open the Old Testament and see that the mystery of Mary is prepared for at every important stage in salvation history. It begins with Sarah, the mother of Isaac, who had been barren, but when she was well on in years and had lost the power of giving life, became, by the power of God, the mother of Isaac and so of the chosen people.
The process continues with Anna, the mother of Samuel, who was likewise barren, but eventually gave birth; with the mother of Samson, or again with Elizabeth, the mother of John the Baptizer. The meaning of all these events is the same: that salvation comes, not from human beings and their powers, but solely from God—from an act of his grace.
(From Dogma und Verkundigung, pp. 375ff; quoted in Co-Workers of the Truth: Meditations for Every Day of the Year [Ignatius Press, 1992], pp. 99-100.)