Why do Catholics pray the Hail Mary?

Catholics pray the Hail Mary because it is rooted in Scripture, honors God's grace working in Mary, and asks for her maternal intercession. It unites two biblical greetings with a petition, expressing the Church's faith that Mary, as Mother of Christ and Mother of the Church, powerfully intercedes for her children.

A Prayer Rooted in Scripture

The address 'full of grace' signals that Mary's holiness is entirely God's gift. The prayer, rightly understood, is itself an act of praise to God for what He accomplished in her.