Mary's role in the Church is inseparable from her union with Christ and flows directly from it. "This union of the mother with the Son in the work of salvation is made manifest from the time of Christ's virginal conception up to his death"; it is made manifest above all at the hour of his Passion:

Thus the Blessed Virgin advanced in her pilgrimage of faith, and faithfully persevered in her union with her Son unto the cross. There she stood, in keeping with the divine plan, enduring with her only begotten Son the intensity of his suffering, joining herself with his sacrifice in her mother's heart, and lovingly consenting to the immolation of this victim, born of her: to be given, by the same Christ Jesus dying on the cross, as a mother to his disciple, with these words: "Woman, behold your son."
-Catechism of the Catholic Church #964