Pastorally so that Catholics would understand Mary's role under Christ. Our Lady's role takes nothing away from Christ. In fact, She is the highest example of association in Her Son's redemption of Mankind. Her mediating role is perfectly in accord with the centrality of Christ. She continues in Heaven in union with Her risen Son.
Mother is associated with the transmission of life.
We all have life in Christ. She is physical mother of Christ and spiritual mother of the members of Christ. She is our Mother not only because She was the physical Mother of Jesus. Mary's faith was first as She gave Her consent to be the Mother of God. And this same faith is what enabled Mary to become the perfect example of the Gospel's meaning of spiritual motherhood.