This question has a limiting atmosphere. That one may pray to Jesus through Mary in no way lessens the direct mediation of our Blessed Savior. We have only one mediator and that is Jesus. Catholics do not substitute Jesus for someone less. Jesus brings our prayers to the Father and brings the Father’s response to us. Chiefly, this is done through the holy sacrifice of the Mass.
Let’s consider the role of “intercessor.” A mediator bridges two sides or two parties. Jesus is both God and man, divine and human. Therefore, He alone is mediator between heaven and Earth, between God and humanity. An intercessor, however, is merely someone who makes a request on behalf of another. Many people in the Gospel interceded for their loved ones to Christ. They asked Him to cure someone else. Intercession does not violate nor dilute mediation. At Mass, through the ministry of the priesthood, the priest is an intercessor on behalf of his people to Jesus. He brings to Jesus the people’s offerings, prayers, and needs. In return, the priest brings to the faithful Jesus’ blessings, gifts, and response. It is in this same way that we understand the intercession of Mary and of all the other saints as well.
In the Nicene Creed, we profess belief in the communion of saints. Community connotes interaction. In heaven, the blessed are in communion around the Godhead in constant adoration, love, and praise. When Catholics pray to a saint, they are not replacing the one Intercessor with the blessed, but rather are praying to the one Intercessor through the blessed. Again, love is not limiting but ever-expanding. It is the love that Jesus has for humanity that allows prayers to come to Him through the blessed.
At Mass, or when someone asks us to pray for them, don’t we offer a prayer or do a good work for their intention? If we can do this, why can’t the saints in heaven do the same for us? Their intercessions in no way reduce the power of God, but ever increase it. Church militant (the faithful on earth) and Church triumphant (the blessed in heaven) are united along with Church suffering (the holy souls in purgatory waiting to enter eternal life). At death, life is changed rather than ended, so there is a wonderful community in the afterlife. The unity of all three churches is most superbly expressed at Mass, when all are united at the altar of praise.
When we love the coheirs and friends of Jesus, we honor God who is their Creator, Redeemer, and Sanctifier; we do this by invoking their prayers. Mary, who is the first of all saints and has a special honor and place in the church, is also our spiritual mother. How fitting it is to go to the mother of God, the mother of the Church, for she is the closest to her Son. Saint Louis de Montfort said beautifully, “To Jesus through Mary.” Mary doesn’t replace Jesus, she augments His importance by uniting us to Him in prayer.