There are many different types of prayer. Praying to the Saints or to Mary is called intercessory prayer. Praying the Our Father or the Hail Mary are called our traditional Catholic prayers are scriptural and taught to us by Jesus Himself. There is also spontaneous prayer, liturgical prayer, communal prayer and private prayer. There are probably more types of prayer that I forgot to mention. But the point is this:
We ask the intercession of the Saints and Mary because they are perfect and holy and I am not. Sin can hinder our prayers and act like a barrier because God is holy and can’t be near sin. Any sin. Remember the story of Adam and Eve and how they had to leave Eden because they disobeyed God so even if I sin a little bit, it hinders my prayers because they are not really being offered up perfectly but when I pray the Rosary and ask Mary to pray for us, since Mary is in Heaven and she is perfect and holy, thru her intercession, my prayer is being offered thru Mary and she delivers my prayer. There is no one who was closer to Jesus than His mother so who among all the saints is more perfect than the Blessed Virgin Mary? That is why when I do pray intercessory prayer, I do thru Mary.
I once heard this explanation of praying the Rosary. Imagine that you are praying to Jesus for the physical healing of your friends cancer and you are praying the Rosary prayer. Jesus is sitting, listening to your prayer request, He too is weeping for your friend. He is also touched at your great faith and how much you love your friend and how much you trust and love His own mother and that you consider her your Heavenly mother. Jesus’ mother is standing behind a curtain and she too is listening to you . She steps out from behind the curtain and says to her son, if not for them, then for me heal her friend’s cancer. I also heard someone say that God gave us Jesus thru Mary and so God is pleased when we pray to Him in Jesus’ name thru Mary.
Psalm 66:18
If I regard iniquity in my heart, the Lord will not hear me.
1 John 1:8-9
If we confess our sins, He is faithful and just and will forgive us our sins and purify us from all unrighteousness.
I think its a good habit to get into that when praying any type of prayer to God, to tell God that I am sorry if I have offended you in any way, thru thought, word or deed and if there is any un-confessed sin in my heart to reveal it so that I may be cleansed from it.