Mary was necessary to God in the Incarnation of the Word

Words to ponder on the Solemnity of the Annunciation of the Lord:

It was through Mary that God the Father gave His Only-begotten to the world. Whatever sighs the patriarchs may have sent forth, whatever prayers the prophets and the saints of the Old Law may have offered up to obtain this treasure for full four thousand years, it was only Mary who merited it and found grace before God (Luke 1:30) by the force of her prayers and the eminence of her virtues. The world was unworthy, says St. Augustine, to receive the Son of God directly from the Father's hands. He gave Him to Mary in order that the world might receive Him through her.

The Son of God became God for our salvation; but it was in Mary and by Mary. God the Holy Ghost formed Jesus Christ in Mary; but it was only after having asked her consent by one of the first ministers of His court. 

God the Father communicated to Mary His fruitfulness, inasmuch as a mere creature was capable of it, in order that He might give her the power to produce His Son and all the members of His Mystical Body.

God the Son descended into her virginal womb as the new Adam into His terrestrial paradise, to take His pleasure there, and to work in secret marvels of grace. 

God made man found His liberty in seeing Himself imprisoned in her womb. He made His omnipotence shine forth in letting Himself be carried by that humble maiden. He found His glory and His Father's in hiding Hs splendors from all creatures here below, and revealing them to Mary only. He glorified His independence and His majesty in depending on that sweet Virgin in His conception, in His birth, in His presentation in the temple, in His hidden life for thirty years, and even in His death, where she was to be present in order that He might make with her but one same sacrifice and be immolated to the Eternal Father by her consent, just as Isaac of old was offered by Abraham's consent to the will of God. It is she who nourished Him, supported Him, brought Him up and then sacrificed Him for us.

~St. Louis De Montfort, True Devotion to Mary