Catholics pray the rosary because it’s a powerful prayer to God, through His mother, Mary.
Praying the rosary has been a tradition in the Church for a long time. It’s a bit fuzzy who made it more formal; some say it was St. Dominic and others say it wasn’t. What really matters is that this prayer is super powerful.
Throughout Church history, many popes and saints have highly recommended that we pray the rosary. St. Louis de Montfort said, “When the Holy Rosary is said well, it gives Jesus and Mary more glory and is more meritorious than any other prayer.”