Praying through Mary is not expressly written in the Bible, it is rather an outcome of a mature understanding of the "Communion of Saints". We the Church believe, as taught by St Paul, that "we are one body, one Spirit in Christ". To the mind of the Church, this has a certain literalness in terms of religious actions. So that when you do good, the entire body is healthy and is affected and when you sin, you affect the entire body. When you pray as well the other members (parts) of this body is equally affected.

Christ came to the world as man and God, establishing his authority and dominion as Lord of the Living, both because he lived, died and resurrected. But also Lord of the dead because through this resurrection he conquered death. Since life springs from him only, and since he has walked the roads of the underworld, he himself has power that penetrates the grave.

This power is also inherited by his Body the Church, so that she is never torn apart or broken by death. NO ! She is actually strengthened by the death of a faithful since her glory shines as more members are glorified.

These glorified members since they are still attached to Christ's body also help in its nourishment. So that when they praise God by chants, or pray to Him, the effects reach the entire body; the effect reach you and me. So even without asking for it, the saints pray for us. And we can raise our hearts to them since we're bound; one body and spirit in Christ to aid us in our needs.