Theologians have debated this for centuries, and I am not theologian, but I will present a VERY SIMPLIFIED and VERY PERSONAL explanation of Marian dogma that I have gleaned from reading A LOT of Marian literature from different periods.
So here goes, and please don't flame, because I'm not presenting this as the official Catholic view. You have to read the Catechism for that.
Because Adam and Eve were both created sinless, and both sinned, both, in a sense, had to be redeemed. Eve ate the apple first, but in the view of Catholic theologians, at least, original sin "didn't count" until Adam ate the apple.
In the same way, Redemption didn't occur until Christ died for our sins on the cross. But Mary's suffering, the "sword that pierced her heart" forecast at the temple where the Presentation took place, along with her willingness to accept the will of God at the Annunciation, contributed to and led to the Redemption, just as Eve's giving into temptation in the garden led to the fall.
That is not to say Mary was a co-redeemper. That is NOT Catholic theology. Many used Revelations 12 to tie this all together.
If there's anyone wondering about Mary's sinlessness -- seriously wondering, that is -- I would highly recommend purchasing the linked book, which is an abridged version of the difficult-to-find Mystical City of God by Sister Maria Agreda. She was a 16th century Spanish nun, who was a mystic.
I haven't read this book, but have read the unabridged version, which costs about $75, if you can find it
Readin it would, imho, convince the doubters. -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Only the word of God is without error..so everything must line up with scripture and be consistant with scripture and be supported with scripture, or it is questionable..
That is what sola scriptura means. That is our final tie breaker on doctrine..
Happy 4th