For when you cannot concentrate, and want something to do with your hands and your mouth while your mind catches up. Twenty scenes from the life of Christ, five at a time, one Hail Mary after another. Nobody prays it perfectly, and that has never mattered.
The repetition is the point, and it is the part people apologize for. Ten Hail Marys in a row, said out loud or under the breath, are not meant to be attended to one by one. They are a floor to stand on. The words carry themselves, the beads keep count so you need not, and what is left of your attention is free to look at the scene — the angel at the door, the garden at night, the empty tomb.
So if your mind wanders, that is not a failure. It is supposed to wander, toward the mystery. If it wanders somewhere else, you bring it back at the next bead and nothing is lost. Twenty minutes of that, most days, is what the Rosary is.
Two histories, and they are kept apart here. The tradition says Mary gave the Rosary to St Dominic in 1208, to preach against heresy; that story is on this site's apparitions index with its standing, and it was promoted by a Breton Dominican, Alan de la Roche, in the 1470s — some two and a half centuries after the event.
The documented history is quieter and older. Monks prayed the hundred and fifty psalms; laypeople who could not read prayed a hundred and fifty Hail Marys instead, on knotted cords, and called it Our Lady's Psalter. That is attested by the twelfth century. Attaching a scene to each group of ten, so the mind had somewhere to go, came in the fifteenth. Pius V fixed the fifteen mysteries in 1569. And in 2002 John Paul II added five more — the Luminous — which makes the Rosary the only major Catholic prayer changed within living memory.
Two years after Pius V fixed it, a fleet sailed against the Ottomans while Rome prayed the Rosary, and won at Lepanto on 7 October 1571. He made the day a feast, and it still is. Leo XIII wrote eleven encyclicals on the Rosary between 1883 and 1898, which is more than any pope has written on almost anything. At Fatima in 1917 the one request repeated at every appearance was to pray it daily.
It is in every Catholic house that has any prayer in it at all, and in a great many pockets. This site's 54-day novena is six Rosaries a week for eight weeks, with the beads drawn on the screen.