The Church honors the canonized saints who are already with the Lord in Heaven because they inspire us by the good example of their lives, and because they can help us by their prayers.
We must honor the saints not just because they can and will pray for us, but also because our love for God demands it. The saints are masterpieces of God’s grace; when we honor them, we are honoring their Maker, their Sanctifier, and their Redeemer. The saints inspire us by the heroic example of their lives. To them we pray, asking their intercession with God for us.
Jesus, having entered into glory and as the eternal High Priest, continues to pray for us. Mary, ever associated with her Son, prays for us with Him. She is not alone in this; the whole community of the blessed in Heaven imitates Christ in their continual concern for us. As we pray for one another upon earth and for the souls in Purgatory, so our brothers and sisters in Heaven intercede for us. We are united with all of them by intimate bonds of Christian love. Mary, our spiritual mother, however, has an altogether exceptional role in this union. Among those who have been redeemed by her Son, her intercessory power is by far the most extensive and effective.
To invoke the intercession of the saints, including Mary, is really to pray that, together with them, we may grow in the love of the triune God, Who wills the Salvation of all; it is to express the longing that the saints, living in personal love of God, will also embrace us in that personal God-given love, and will, by their prayers, assist us in obtaining benefits from God.
When we honor the canonized (and uncanonized) saints, we are honoring many of our own loved ones who now are with God in Heaven, because every soul in Heaven is a saint.