I think it is a development of Catholic intuition about God that is distinct from NT theologies. Later on came the understanding that grace was also mediatated through sacred images. I note that the Catholic poet in English most viscerally aware of grace being mediated in the world, Gerard Manley Hopkins, is also the one to have written some of the greatest Marian Verses. What I am saying then is that mediation of grace is the defining aspect of Catholic Christianity. A Catholic, caught in a swirl of angels, saints, fellow humans, sacraments and so on can never be alone before God.
All people can mediate grace.
What is special about Mary is the very strong perception that given her place in the economy of salvation she is the greatest of all saints, to whom too many honors cannot be given. These are two visions of Christianity. It may be weaker, but I prefer the second.