Mary has been petitioned by the faithful since the times of the early Church. First, as the Bible teaches (particularly in Luke's Gospel) because the Blessed Virgin of Nazareth is the mother of Jesus Christ, Son of God, who became man in her virginal womb for the salvation of all mankind. Secondly because Mary is also the mother of us all, since the moment at Christ’s Crucifixion He himself entrusted the apostle John to her, and through him all the human beings who confess themselves to be God’s children. In this way, Jesus made His mother our universal Mother (cf Jn 19: 26).
A universal, tenderly beloved Mother
Still today, anywhere in the world and in just about every language, men and women across the line of every socio-cultural and economical border pray to the Virgin of Nazareth as they would a tenderly beloved mother. To give the full measure of Marian devotion in Christianity, one only has to take a look at her importance in the Catholic and Orthodox liturgies, the great hymns from the earliest tradition in the Church (the well-loved 6th century Akathist Hymn), or the most famous Marian prayer of all times, the Holy Rosary, or the Magnificat, which Martin Luther himself commented to the glory of the Virgin Mary, or the countless artistic renditions of Mary, including those that take Mary as their main theme (among them the world renowned Byzantine icons).
Mary is venerated; God alone is adored
However the devotion Christians give Mary is of a different nature than the devotion they give God Himself: Mary isn't a divinity that they adore. Adoration is for God alone. Mary is a creature whom they venerate. Yet, this sentiment of veneration is superior to that given all the other saints, for the Blessed Virgin Mary is really the only one among God's creatures who can truly bear these titles of glory: Mother of God, Mother of the Redeemer, Mother of the Church, Queen of Heaven and Earth, etc.
To paraphrase many saints and Doctors of the Church, Mary doesn't keep for herself the children whom she protects. On the contrary, she never ceases to lead them to her Son, to the point that one could outline the finality of the devotion to Mary in the Church in these few words from the French priest Saint Louis Grignion de Montfort:
"To Jesus Christ through Mary."