Objection 1. It would seem that it was unnecessary to announce to the Blessed Virgin that which was to be done in her. For there seems to have been no need of the Annunciation except for the purpose of receiving the Virgin's consent. But her consent seems to have been unnecessary: because the Virginal Conception was foretold by a prophecy of "predestination," which is "fulfilled without our consent," as a gloss says on Matthew 1:22. There was no need, therefore, for this Annunciation.
Objection 2. Further, the Blessed Virgin believed in Incarnation, for to disbelieve therein excludes man from the way of salvation; because, as the Apostle says (Romans 3:22): "The justice of God (is) by faith of Jesus Christ." But one needs no further instruction concerning what one believes without doubt. Therefore the Blessed Virgin had no need for the Incarnation of her Son to be announced to her.
Objection 3. Further, just as the Blessed Virgin conceived Christ in her body, so every pious soul conceives Him spiritually. Thus the Apostle says (Galatians 4:19): "My little children, of whom I am in labor again, until Christ be formed in you." But to those who conceive Him spiritually no announcement is made of this conception. Therefore neither should it have been announced to the Blessed Virgin that she was to conceive the Son of God in her womb.
On the contrary, It is related (Luke 1:31) that the angel said to her: "Behold, thou shalt conceive in thy womb, and shalt bring forth a son."
I answer that, It was reasonable that it should be announced to the Blessed Virgin that she was to conceive Christ. First, in order to maintain a becoming order in the union of the Son of God with the Virgin--namely, that she should be informed in mind concerning Him, before conceiving Him in the flesh. Thus Augustine says (De Sancta Virgin. iii): "Mary is more blessed in receiving the faith of Christ, than in conceiving the flesh of Christ"; and further on he adds: "Her nearness as a Mother would have been of no profit to Mary, had she not borne Christ in her heart after a more blessed manner than in her flesh."
Secondly, that she might be a more certain witness of this mystery, being instructed therein by God.
Thirdly, that she might offer to God the free gift of her obedience: which she proved herself right ready to do, saying: "Behold the handmaid of the Lord."
Fourthly, in order to show that there is a certain spiritual wedlock between the Son of God and human nature. Wherefore in the Annunciation the Virgin's consent was besought in lieu of that of the entire human nature.
Reply to Objection 1. The prophecy of predestination is fulfilled without the causality of our will; not without its consent.
Reply to Objection 2. The Blessed Virgin did indeed believe explicitly in the future Incarnation; but, being humble, she did not think such high things of herself. Consequently she required instruction in this matter.
Reply to Objection 3. The spiritual conception of Christ through faith is preceded by the preaching of the faith, for as much as "faith is by hearing" (Romans 10:17). Yet man does not know for certain thereby that he has grace; but he does know that the faith, which he has received, is true.
Source: Summa Theologica (New Advent)