In several ways: by her conception of the One Who is Truth, she bears unique witness to the Truth and in her very person is a sign against those who would deny that the Second Person of the Trinity became flesh (the source of all heresy); furthermore, by her role as spouse of the Holy Spirit and mediatrix of grace she obtains for the Church the graces to bear witness to the Truth and to call out heresy when it is seen.
This is not obscure theology; it is very practical. Have you ever noticed how all modern heresies have one thing in common? They all leave out Mary. Let's look at a few. Protestant-influenced biblical criticism sees Marian devotion as a superstitious accretion from the Middle Ages; post-Vatican II modernist theology has downplayed Marian devotion to the point that (while the Rosary has not disappeared) Marian devotions like the Angelus, May Crownings, the Litany of Loreto and others are almost unknown. Feminist theology sees Mary as a sign of the submissive, male-dominated woman who must not be emulated but scorned. Protestantism has a long and well-known aversion to Mary because they fear that one commits idolatry in honoring her. Liberation theology has no place for Marian devotion because Marian devotion always encourages humility and submission, not revolution and envy. All of these heresies have one thing in common: they reject traditional Marian devotion.