Because that's the point on which Protestant friends and interlocutors always get hung up--how could Mary need the salvation offered by Jesus if she never sinned?
And the answer, of course, is that she was conceived immaculate and never sinned by the grace which came through her Son and Savior.
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.The order of events is unusual, yes, and yet Jesus, the Second Person of the Blessed Trinity, is eternal, and so all events in his personal experience are similarly eternal--standing outside of time.
All grace comes through Jesus Christ. No grace enters time and history except through Jesus Christ, including all grace in the lives of the Old Testament folk, all grace in the lives of people who've never heard the Gospel, and all grace which enters the lives of Christians--even though Jesus comes later in time than the folk of the Old Testament.
All grace comes to Mary through Jesus, just as much and more than the grace which comes into the lives of anyone else.
My soul proclaims the greatness of the Lord;It's all in Doctor Who, all in Doctor Who--goodness me, what do they teach in schools these days?
my spirit rejoices in God my savior.
For he has looked upon his handmaid’s lowliness;
behold, from now on will all ages call me blessed.
The Mighty One has done great things for me,
and holy is his name.
His mercy is from age to age
to those who fear him.
He has shown might with his arm,
dispersed the arrogant of mind and heart.
He has thrown down the rulers from their thrones
but lifted up the lowly.
The hungry he has filled with good things;
the rich he has sent away empty.
He has helped Israel his servant,
remembering his mercy,
according to his promise to our fathers,
to Abraham and to his descendants forever.