The Rosary is Mary's favorite prayer because it provides a time for meditation about her Son's Divine life here on earth.  There are many, many blessings associated with the Rosary. A great resource for information abouthe  rosary is the Rosary Center & Confraternity run by the Dominican priests: https://rosarycenter.org/ 

According to tradition, St. Dominic was distressed at his lack of success in his preaching against a heresy that was spreading during the late 12th and early 13th centuries. In his desperation he turned to the Mother of God for help. She appeared to him and told him to use her Psalter in conjunction with his preaching of the mysteries of our salvation, as an instrument in combatting the great heresy of his day.

The custom of counting repeated prayers by the use of a string of beads or knots, or pebbles in a bowl was prevalent long before the time of St. Dominic. This was common among the Moslems, the Buddhists, and other non-Christian religions as well as among Christians.

From time immemorial the 150 psalms of the bible comprised the most important part of the official liturgical prayers prayed by the clergy and the monks in monasteries. Since, however, many of the common folk were illiterate, there was an attempt to offer those who could not read  a substitute for the 150 psalms. The practice arose of substituting 150 OUR FATHERS in place of the Latin psalms, using a string of beads to count them, dividing them into “fifties”. This chaplet, or string of beads, came to be known as “Paternoster” beads. Little by little, the HAIL MARY took its place along side the CREED and the OUR FATHER as a standard prayer. But still, it was only the first half that was used. In the course of time there came to be a parallel Psalter, i.e., one of 150 HAIL MARYS known as the MARIAN PSALTER.