I was asked why Catholics pray the Hail Mary, when Jesus tells us to pray the Our Father. He thought this was disobeying God. Here’s my reply:
Answer: Jesus gives us a model prayer, but I don’t know any Christian who only prays the Our Father. We pray the Our Father at every Mass (we don’t pray a Hail Mary at every Mass, in fact, pretty rarely in Mass). Every single Catholic church around the world anywhere prays the Our Father at every Mass, and most of these churches are open and worshiping 7 days a week. We pray the Our Father constantly. So we follow God’s commandment there – we don’t disobey.
Your real question seems to be, “why pray the Our Father AND the Hail Mary,” or “why pray to God but also to Mary and the saints?” This is a good question, but one which I think we get some guidance from the Bible on. As Catholics, we think that when you’re in Heaven, God permits you to know what’s going on down here on Earth. Moses and Elijah show up at the Transfiguration, and in Luke 9:30-31 talk to Jesus about His upcoming crucifixion. So the Saints in Heaven are aware of what is going on down here. In Luke 16:24, Jesus tells a parable that has Lazarus praying to Abraham… and Jesus calls him “Father Abraham” in the story (this will be relevant to your later question).
Even though we pray to the saints, we don’t worship them. Prayer is just the form of communication we have to those in Heaven – we submit our prayers to God. If I want to tell someone next to me to pray for me, I can just tell them verbally. If I want to tell you to pray for me, I need to send you a message electronically. If I want to tell someone who isn’t alive to pray for me, I just say it outloud, and God allows them to know what I’ve said. That’s prayer, and through it, the communion of saints, living and dead, stay together, praying for one another.
I imagine your reaction might be, “But even if the saints can hear prayers, why pray to them? I mean, all we’re doing is asking them to pray to God for us?” Well, James 5:16 says, “Therefore confess your sins to each other and pray for each other so that you may be healed. The prayer of a righteous man is powerful and effective.” It tells us both to confess to one another, AND it tells us that intercessory prayers are powerful. So yes, all Catholics who go to church pray to God: He’s the summit and focal point of our Faith. But don’t forget that Paul calls us to intercede for one another in 1 Timothy 2:1 and Ephesians 6:18. And we see the Angels in Heaven interceding in Revelation 5:8, and 8:3-4.
We’re supposed to pray for one other, and we’re supposed to tell one another what we need prayed for (or, at least, just say, “I need you to pray for me”). In the Hail Mary, we have two parts. The first half is taken straight from the Bible: Luke 1:28, Luke 1:42. In Luke 1:42, we see the Holy Spirit inspiring Elizabeth to proclaim Mary’s Blessedness. The second half is our telling Mary, “we need you to pray for us.” We don’t think she has powers apart from God, but we believe that Jesus, her Son, cares about her and listens to her. In John 2, she persuades Jesus to perform His first miracle, even though He initially says, “My hour is not yet come.” So we pray to all the saints, and in a special way to Mary, to pray for us and to petition God for us, because they’re in Heaven, so they’re fully righteous.