Protestant
Why do Catholics pray to saints?
Catholics ask saints to intercede for them, the same way one might ask a friend to pray for them — the saints in heaven are alive in Christ and their prayers avail much.
Catholic teaching
The Church teaches that death does not sever the Body of Christ; the saints in glory continue to love and intercede for those on earth. Asking their intercession is not worship (latria), which is due to God alone, but the veneration (dulia) fitting to friends of God.
Biblical foundation
Revelation shows the elders in heaven offering the prayers of the saints to God (Rev 5:8), and James teaches that the prayer of a righteous man avails much (Jas 5:16). If the living can intercede, how much more the perfected righteous in heaven.
Scripture references
Historical evidence
Third-century catacomb inscriptions already ask deceased Christians to pray for the living, showing the practice predates any medieval development by centuries.
The Church Fathers
“A Christian people celebrates together in religious solemnity the memorials of the martyrs, both to encourage their imitation and to be associated with their merits and helped by their prayers.”