God chose this means of coming into the world, therefore it is now essential for our salvation. Mary is the channel, the conduit through which He comes into the world, takes human flesh, and redeems mankind.
The Fathers of the Church tell us that the womb of Mary is like a mold ... in which she formed Christ in His humanity by the will of the Father, by the will of God.
If she was chosen among all women to form the body of Christ for us, for our salvation, then she continues to this day to form that same image in her womb in each one of us. We're called to be members of the Body of Christ through Baptism, formed into the image of Jesus Christ our Lord and Savior.
The Lord isn't going to create a new way, a new channel, a new mode of creating His image in children. He's going to use the one He designed perfectly two thousand years ago. That's why ever since, the Church has always recognized her not only as the Mother of God, as the Mother of Christ, but our Mother. The Mother of the Church. The Mother of each individual disciple of Jesus Christ.
She is necessary, not because God couldn't have done it another way, but because He chose Her as His singular instrument to form Christ in the flesh.
The Church Fathers say that one of the marks of salvific grace in the soul of a Christian is true and abiding devotion to the Mother of God.