Immaculate Conception: Why was it necessary for our Salvation?

According to many Catholic apologists, like Dr. Robert Sungenis,1 the dogma of the immaculate conception of Mary is not simply binding because it is a divinely revealed dogma, but according to them it is binding additionally because logically speaking without Mary being sinless Jesus could not accomplish the redemptive work in the first place. **What exactly do they mean by this?** The story goes like this, to save mankind Jesus had to be sinless. Therefore Mary had to be sinless to not pass on original sin. But then Protestant apologists just say "the grace could have been given directly to Jesus" or "why wasn't the grace given to the mother and father of Mary, or even further back?" > Accordingly, by the inspiration of the Holy Spirit, for the honor of > the Holy and undivided Trinity, for the glory and adornment of the > Virgin Mother of God, for the exaltation of the Catholic Faith, and > for the furtherance of the Catholic religion, by the authority of > Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by > our own: > > **We declare, pronounce, and define that the doctrine which holds that > the most Blessed Virgin Mary, in the first instance of her conception, > by a singular grace and privilege granted by Almighty God, in view of > the merits of Jesus Christ, the Savior of the human race, was > preserved free from all stain of original sin, is a doctrine revealed > by God and therefore to be believed firmly and constantly by all the > faithful.** Hence, if anyone shall dare -- which God forbid! -- to think > otherwise than as has been defined by us, let him know and understand > that he is condemned by his own judgment; that he has suffered > shipwreck in the faith; that he has separated from the unity of the > Church; and that, furthermore, by his own action he incurs the > penalties established by law if he should are to express in words or > writing or by any other outward means the errors he think in his > heart.” > > **Blessed Pope Pius IX, Ineffabilis Deus, 8 December 1854** Questions in formal form is as follows: **Why was it necessary for the salvation plan to have specifically Mary the mother of Jesus be utterly without sin?** ---------- 1. For example, in this [video debate](https://www.youtube.com/watch?v=Wus6CrzleRw%22) at 1:05:18 ("because if she isn't sinless, she's going to produce a sinful Savior") and 1:13:00.