St. Bernard of Clairvaux spoke of Mary’s Fiat (her “be it done to me according to your word,” her “yes”) as a moment that the whole world was waiting on her reply. This, because he says that God needed Mary’s freely given Yes in order to bring Jesus into the world, Word made flesh.
Pope Benedict XVI elaborates:
Certainly, this Yes Mary says is wholly by grace. The dogma of the Immaculate Conception of Mary, whereby she was freed from original sin, has in fact just exactly this as its sole significance: that no human being can set in motion the process of salvation by his own powers alone, but that his Yes is wrapped around and supported within by that divine love which comes first and before all else that already surrounds man before he is even born. “All is grace.” Yet grace does not remove freedom; rather it brings it into being. The entire mystery of redemption is present in this story and it is summed up in the figure of the Virgin Mary: “Behold, I am the handmaid of the Lord; let it be to me according to your word” (Luke 1:38).
-Pope Benedict XVI in God is Near Us: The Eucharist, The Heart of Life