Why do we need to say the Rosary
As St Louis de Montfort said that there are two reasons for doing this: first of all, to honour the three crowns of Jesus and Mary - Jesus' crown of grace at the time of his Incarnation, his crown of thorns during his passion, and his crown of glory in heaven, and of course the three-fold crown which the Blessed Trinity gave Mary in heaven. Secondly, we should do this so that we ourselves may receive three crowns from Jesus and Mary, the first a crown of merit during our lifetime; the second, a crown of peace at our death; and the third, a crown of glory in heaven.
Long time ago, there was a woman in Rome who was so devout by her holy life that she put even the strictest religious to shame. One day, she went to St Dominic to consult with him about her spiritual life and asked him to hear her confession. For penance he gave her one Rosary to say and counselled her to say a Rosary everyday. She immediately began to make excuses...St Dominic urged her again to take his advice but she would not hear of it. She left the confessional almost scandalised by the advice of this new spiritual director. Later, when she was at prayer, she fell into an ecstasy and had a vision of her soul appearing before our Lord's Judgement Seat. St Michael put all her penances and other good works on one side of the scales and all her sins and imperfections on the other. The side of her good works was greatly outweighed by the side of her sins and imperfections. Terrified, she cried for mercy.
She implored the help of Mary, who took the one Rosary she had said for her penance and dropped it on the side of her good works. This one Rosary was so heavy that it outweighed both her sins and her good works. Mary then reproved her for refusing to follow the advice of St Dominic and for failing to say the Rosary everyday. As soon as she came to herself, she rushed to throw herself at St Dominic's feet, told him what happened, asked his forgiveness for her unbelief and promised to say the Rosary everyday. By this means she rose to Christian perfection and then to the glory of everlasting life.
(Reference: The Secret of the Rosary, St Louis Grignion de Montfort, Catholic Book Publishing Corp., N.J. 2004)