To answer “why Catholics pray the Rosary” we need only read the following (from Wikipedia):

“Devotion to the Rosary is one of the most notable features of popular Catholic spirituality. Pope John Paul II placed the Rosary at the very center of Christian spirituality and called it ‘among the finest and most praiseworthy traditions of Christian contemplation.’ Catholics believe the Rosary is a REMEDY AGAINST SEVERE TRIALS, TEMPTATIONS AND THE HARDSHIPS OF LIFE, and that the Rosary is ONE OF THE GREAT WEAPONS GIVEN TO BELIEVERS IN THEIR BATTLE AGAINST EVERY EVIL. Pope Pius XI is quoted as saying, for example, ‘The Rosary is a powerful weapon to PUT DEMONS TO FLIGHT.’”

As you examine these statements by the Word of God, you will see that the Romans Catholic Church has replaced the Word of God, which alone is the “remedy against severe trials, temptations and the hardships of life” (see Romans 15:4) and a “powerful weapon to put demons to flight” (see Ephesians 6:10-17), with a man-made prayer that is filled with errors.

The Rosary is made up of various prayers and the reciting of the Apostles’ Creed, but the most well-known part of the Rosary is “the Hail Mary” prayer. I will quote this and then compare it with Scripture.

“Hail Mary, full of grace, the Lord is with thee. Blessed art thou among women, and blessed is the fruit of thy womb, Jesus. Holy Mary, Mother of God, pray for us sinners, now and at the hour of death. Amen.”

There are FOUR serious ERRORS in this prayer. 1) Mary is NOT FULL OF GRACE. Scripture never teaches us that Mary can minister grace to us in our time of need. Instead, we read that Jesus Christ is FULL OF GRACE in John 1:14 & 16, “And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, FULL OF GRACE and truth…and of His fullness we have all received, and GRACE FOR GRACE.”

2) Though Mary truly was “blessed among women” (Luke 1:28 & 42), it is wrong to refer to her as “HOLY Mary.” Catholics believe that Mary was SINLESS yet Mary herself said these words in Luke 1:47, “And my spirit has rejoiced in God my SAVIOR.” Mary knew that she was, like everyone else born into the human race (except Jesus), a SINNER who needed a SAVIOR.

3) Mary is NOT “the MOTHER OF GOD.” She is indeed the “mother of Jesus,” (for she was the human vessel that the Holy Spirit used to bring about the birth of Jesus Christ…Luke 1:26-35), but she could not be called the mother of God for God is eternal (Psalm 90:2; 1st Timothy 6:13-16) and had no beginning.

4) It is blasphemy to say to Mary “pray for us sinners.” Mary is NOT the sinner’s MEDIATOR and ADVOCATE! These titles belong to Jesus Christ alone, as we see in 1st Timothy 2:5 and 1st John 2:1, “For there is one God and one Mediator between God and men, the Man Christ Jesus…My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous.” Is this not clear? Jesus is the one who stands between the sinner and God as our blessed Mediator and Advocate. To put Mary in that place is indeed blasphemy! The fact is the last time we read of Mary in Scripture she is seen at a prayer meeting and instead of sinners “praying to Mary,” Mary is joining in prayer with the others as they “prayed to God.” Acts 1:14 reads, “These all continued with one accord in prayer and supplication, with the women and Mary the mother of Jesus, and with His brothers.”  (466.1)  (DO)