What does the perpetual virginity of Mary add to our salvation? I tried this question earlier and it was closed due to so many other similar questions but, having looked through I didn't find an answer.

What, if any, is the salvific benefit of the perpetual virginity of Mary?  In other words, having been born of a virgin according to prophesy and all other parts of the life, death, burial, resurrection, ascension, and coming again of Christ remaining the same, is our salvation in Christ lessened in some way if Mary has moral and lawful intercourse with her husband?
Ken Graham’s fulsome answer deals almost entirely with the ‘perpetual virginity’ of Mary but I am concerned to look at the claimed role of Mary in salvation because, whether or not Mary remained a virgin after giving birth to Jesus Christ, your question deals with a biblical doctrine – salvation – whereas there is no biblical doctrine of the perpetual virginity of Mary. The Bible is at pains to stress that Mary was a virgin when the miraculous conception took place, but is silent on such intimate matters with regard to Mary thereafter. It also has nothing to say on Mary having any part, or role, whatsoever in salvation. We may misunderstand how salvation is bestowed if we misunderstand Mary’s biblical place in the divine scheme of things. There is no mention of her being in any way involved in salvation in the early Church.

From a Roman Catholic “Encyclopedia of Theology” edited by Karl Rahner, pages 896-7, under the heading ‘Mariology’ it is shown that Mary is contrasted with Eve. As Eve's disobedience brought ruin, the faith and obedience of Mary brought salvation. Mary's claimed role in the history of salvation is compared with that of the Catholic Church. The absolute sinlessness of Mary was taught for the first time by Pelagius and Augustine, then expanded into the freedom from original sin which was then attributed to Mary. Catholic teaching is that Mary plays a vital role in bringing salvation to sinners. This Catholic Encylopedia which I consulted states that Mary:


  “…became, as Pius XII said (Constitution Ad caeli reginam, 11 October
  1954: DS 3913-17), “Queen of Heaven”. This title… indicates the lofty
  position of Mary in the divine economy and in the historical course of
  salvation…
  
  Mary entered into the process of salvation through her faith… By her
  Fiat to the divine message Mary contributed to salvation, just as Eve
  had done to man’s ruin (Lumen Gentum, art. 56). This does not mean
  that God made his plan of salvation dependent on Mary’s consent but
  that according to the eternal plan of salvation man for his part was
  to assent to his salvation, through divine grace. Humanity’s Yes to
  God and to Christ the saviour is summed up in Mary. In her Fiat of
  faith, she received salvation for all...   Mary was the primary
  recipient of this salvation, which she took to herself in the most
  excellent way, not only for herself in individualistic isolation but
  with a willingness and an openness which were orientated to all men.
  Her personal appropriation of salvation has ecclesiastical
  significance. The salvation of Christ is concretely embodied in the
  sacrament of the Church, as Lumen Gentium affirms (art. 59). Salvation
  is present and accessible in the Church, and Mary is the first and
  most privileged member of the Church…
  
  Vatican II avoided speaking of her universal mediation of grace. But
  the truth behind such terms is taught with reserve, while the
  mediatorship of Christ is strongly stressed and all Mary s activity is
  seen exclusively in the perspective of Christ. If, nonetheless, the
  mediatorship of Mary is affirmed, this is in order to bring out a
  fundamental thought from the Bible, the solidarity of all men. Men do
  not receive salvation as individuals, or monads in isolation from each
  other, but as social beings. Each one who receives the gift of
  salvation becomes himself a source of salvation. The good of one is
  fertile in good things for the other. This general principle holds
  good for Mary in a special and comprehensive way. Hence Mary’s
  mediatorship is to be understood on the level of the solidarity of all
  mankind which is in need of redemption, to which she herself belongs,
  and not on that of the one and only saviour.
  
  In the light of the thesis that Mary's glorified existence is that of
  intercession, and that this is essential for her, the much discussed
  question of whether Mary's function is sacramental or petitionary
  seems to be given too much superficial prominence... The function of
  Mary in salvation determines her relation to the Church... Mary is
  mother of the Church under this more individualistic aspect, since she
  is effectively concerned for the salvation of each individual.” (pages
  898-901)


It is seen that arriving at the teaching of Mary’s role in salvation is dependent on many stepping stones being added over time to the doctrine of Mary. There is no mention of her being in any way involved in salvation in the early Church. Only at the beginning of the 3rd century did Hippolytus of Rome give her the title of Deipara (the bringer-forth of God). That was changed later to Theotokos (Mother of God), the Encyclopedia commenting that:


  “It was a term which brought out better than Deipara the fact that
  Mary’s function was not merely physiological but also spiritual and
  personal. It paved the way for the concept of Mary as the spiritual
  mother of all the faithful… As regards the perpetual virginity of Mary
  there was no fully general consensus before the Council of Ephesus.
  [431]” (p 896)


Whatever part Mary’s claimed perpetual virginity plays in any role of salvation, it could not have been significant if it was not settled by Catholic theologians until 431. It follows that if that, and any other stepping-stones along the path of Mary’s elevation to eventually being viewed as a co-Redemptrix in salvation is misplaced, there will be a need to ask more questions.