I’ve read a couple books on the theology of Mary (Mary In the Early Church, Mary Throughout the Ages) and what hasn’t been said above, with enough emphasis, is that the role that Mary had in her earthly life is the role she is attributed to have now.
So, for example, on the cross, when Jesus said to the apostle John “behold your mother” that pertained then to John and now to us all. Like it or not, that’s how theology seems to work.
And, to the title question, since Jesus came through Mary, by childbirth, so salvation and graces flow to us through Mary. That’s theology. It’s an interpretation of scripture and an application to us today.
If I understand what I read about Mary’s Immaculate Conception, Thomas Aquinas did not believe in it (it was not a settled doctrine of the church in his day); he believed, instead, that she was made immaculate the moment after her conception. The dogma being promulgated only in 1854, there is a long history of debate on the question.
Duns Scotus is the theologian who said it was NECESSARY for Mary to be IC in order to conceive and bear Jesus. It took 500 years for this to “sink into” the Church before the doctrine was promulgated.
By “necessary” I personally understand that it was God’s will that it be this way. In the “big picture” of the Bible, Mary represents a new beginning for the human race, just as Adam and Eve were immaculately created. So, Mary’s IC represents a new order, a new beginning, a restoration of mankind to the status of Eden, through whom Jesus arrives.
We cannot give any more honor to God or to Mary that God has already established. So, the IC is not for creating greater honor for Mary. It was, instead, for defining exactly what occurred in the incarnation and nativity of Jesus.