What does the perpetual virginity of Mary add to our salvation?
It makes Mary the Mother of the Church and the spiritual Mother of all mankind. The perpetual virginity of Mary is a complement to the redemptive work of her Son, Jesus who is called the Christ. The Catechism of the Catholic Church speaks thus about Mary’s virginity and motherhood:
Mary's divine motherhood
495 Called in the Gospels "the mother of Jesus", Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as "the mother of my Lord". In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father's eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly "Mother of God" (Theotokos).
496 From the first formulations of her faith, the Church has confessed that Jesus was conceived solely by the power of the Holy Spirit in the womb of the Virgin Mary, affirming also the corporeal aspect of this event: Jesus was conceived "by the Holy Spirit without human seed". The Fathers see in the virginal conception the sign that it truly was the Son of God who came in a humanity like our own. Thus St. Ignatius of Antioch at the beginning of the second century says:
You are firmly convinced about our Lord, who is truly of the race of David according to the flesh, Son of God according to the will and power of God, truly born of a virgin,. . . he was truly nailed to a tree for us in his flesh under Pontius Pilate. . . he truly suffered, as he is also truly risen. The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility: "That which is conceived in her is of the Holy Spirit", said the angel to Joseph about Mary his fiancee. The Church sees here the fulfillment of the divine promise given through the prophet Isaiah: "Behold, a virgin shall conceive and bear a son."
498 People are sometimes troubled by the silence of St. Mark's Gospel and the New Testament Epistles about Jesus' virginal conception. Some might wonder if we were merely dealing with legends or theological constructs not claiming to be history. To this we must respond: Faith in the virginal conception of Jesus met with the lively opposition, mockery or incomprehension of non-believers, Jews and pagans alike; so it could hardly have been motivated by pagan mythology or by some adaptation to the ideas of the age. The meaning of this event is accessible only to faith, which understands in it the "connection of these mysteries with one another" in the totality of Christ's mysteries, from his Incarnation to his Passover. St. Ignatius of Antioch already bears witness to this connection: "Mary's virginity and giving birth, and even the Lord's death escaped the notice of the prince of this world: these three mysteries worthy of proclamation were accomplished in God's silence."
Mary - "ever-virgin"
499 The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity even in the act of giving birth to the Son of God made man. In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it." And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin".
500 Against this doctrine the objection is sometimes raised that the Bible mentions brothers and sisters of Jesus. The Church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, "brothers of Jesus", are the sons of another Mary, a disciple of Christ, whom St. Matthew significantly calls "the other Mary". They are close relations of Jesus, according to an Old Testament expression.
501 Jesus is Mary's only son, but her spiritual motherhood extends to all men whom indeed he came to save: "The Son whom she brought forth is he whom God placed as the first-born among many brethren, that is, the faithful in whose generation and formation she co-operates with a mother's love."
Mary's virginal motherhood in God's plan
502 The eyes of faith can discover in the context of the whole of Revelation the mysterious reasons why God in his saving plan wanted his Son to be born of a virgin. These reasons touch both on the person of Christ and his redemptive mission, and on the welcome Mary gave that mission on behalf of all men.
503 Mary's virginity manifests God's absolute initiative in the Incarnation. Jesus has only God as Father. "He was never estranged from the Father because of the human nature which he assumed. . . He is naturally Son of the Father as to his divinity and naturally son of his mother as to his humanity, but properly Son of the Father in both natures."
504 Jesus is conceived by the Holy Spirit in the Virgin Mary's womb because he is the New Adam, who inaugurates the new creation: "The first man was from the earth, a man of dust; the second man is from heaven." From his conception, Christ's humanity is filled with the Holy Spirit, for God "gives him the Spirit without measure." From "his fullness" as the head of redeemed humanity "we have all received, grace upon grace."
505 By his virginal conception, Jesus, the New Adam, ushers in the new birth of children adopted in the Holy Spirit through faith. "How can this be?" Participation in the divine life arises "not of blood nor of the will of the flesh nor of the will of man, but of God". The acceptance of this life is virginal because it is entirely the Spirit's gift to man. The spousal character of the human vocation in relation to God is fulfilled perfectly in Mary's virginal motherhood.
506 Mary is a virgin because her virginity is the sign of her faith "unadulterated by any doubt", and of her undivided gift of herself to God's will. It is her faith that enables her to become the mother of the Savior: "Mary is more blessed because she embraces faith in Christ than because she conceives the flesh of Christ."
507 At once virgin and mother, Mary is the symbol and the most perfect realization of the Church: "the Church indeed. . . by receiving the word of God in faith becomes herself a mother. By preaching and Baptism she brings forth sons, who are conceived by the Holy Spirit and born of God, to a new and immortal life. She herself is a virgin, who keeps in its entirety and purity the faith she pledged to her spouse."
Mary desires no less than the salvation of all sinners. In this sense her spiritual maternity is a compliment to her Son’s redemptive actions on the Cross. Mary desires that the fruit of her Divine Son’s Passion and Death be to the greatest possible efficaciousness.
We should remember that it was her Divine Son, Jesus, who gave us the eminent gift of his Mother even as he was dying on the cross: “Then he said to the disciple, ‘Here is your mother’” (John 19:27).
Saint John Paul II reminds us, “From the time when ‘the disciple took her to his own home,’ the mystery of the spiritual motherhood of Mary has been actualized boundlessly in history. Motherhood means caring for the life of the child. Since Mary is the mother of us all, her care for the life of man is universal” he said.
We can all understand the universal care of our Mother in Heaven for us or, at the very least, the longing for that spiritual motherly care in our lives. The Blessed Mother’s love and care is especially healing for those whose own biological mothers were unwilling or unable to provide authentic self-giving love. Mother Mary loves us all with a pure, selfless, and steadfast motherly love. She tirelessly calls to our hearts and souls. Her utmost aim is to bring us to her Son.
Mother Mary’s care is rooted in the Holy Spirit. Saint John Paul II tells us, “The care of a mother embraces her child totally. Mary’s motherhood has its beginning in her motherly care for Christ. In Christ, at the foot of the cross, she accepted John, and in John she accepted all of us totally. Mary embraces us all with special solicitude in the Holy Spirit. For as we profess in our Creed, he is ‘the giver of life.’ It is he who gives the fullness of life, open towards eternity” he explained. - Mary, Mother to us all
In closing, I would simply like to add the words that Mary spoke at the Wedding Feast:
On the third day there was a wedding in Cana of Galilee, and the mother of Jesus was there. 2Jesus and his disciples had also been invited to the wedding. 3When the wine gave out, the mother of Jesus said to him, “They have no wine.” 4And Jesus said to her, “Woman, what concern is that to you and to me? My hour has not yet come.” 5His mother said to the servants, “Do whatever he tells you.” 6Now standing there were six stone water jars for the Jewish rites of purification, each holding twenty or thirty gallons. 7Jesus said to them, “Fill the jars with water.” And they filled them up to the brim. 8He said to them, “Now draw some out, and take it to the chief steward.” So they took it. 9When the steward tasted the water that had become wine, and did not know where it came from (though the servants who had drawn the water knew), the steward called the bridegroom 10and said to him, “Everyone serves the good wine first, and then the inferior wine after the guests have become drunk. But you have kept the good wine until now.” 11Jesus did this, the first of his signs, in Cana of Galilee, and revealed his glory; and his disciples believed in him. - John 2:1–2:11
Mary’s unique desire is to send us towards Jesus.
By the consecration of her perpetual virginity Mary’s motherhood not only encompasses her Son , but also of His Church and all mankind.
What if Mary had moral and lawful intercourse with her husband, St. Joseph?
Even without the perpetual virginity of Mary, our salvation in Christ not lessened! Christ would still be Our Redeemer and Mary would still be the Mother of Jesus, the Church founded by Christ, and all mankind. However, it would be seen through the eyes of the Church in a totally different perspective!
The opposite perspective would not diminish the the sanctity of either the redemptive mission of Christ or of Jesus himself. Nor would it diminish the sanctity of the Mary and her role in her intercession on behalf of the mystical body of Christ which is called the Church.