Over the centuries, there have been thousands of reported Marian apparitions. Very few have been officially approved by the Church. Even those that are approved (including Guadalupe and Lourdes) are considered “private revelation.” In other words, they aren’t a necessary component of a Catholic faith.
That said, Marian apparitions have reenergized the faith of many Catholics. Apparitions have led to renewed prayer and repentance (Fatima), to the building of a church (Guadalupe), and to physical healing (Lourdes). Typically, Mary appears to children, the poor, or the marginalized. In doing so, she reaffirms the dignity of the powerless.
In 1996, Cardinal Joseph Ratzinger – who later became Pope Benedict XVI – was interviewed by Portuguese radio about Mary’s apparitions at Fatima. He explained that Mary did not appear “in order to cause a sensation, but to recall the world, by means of simple people, to simplicity, that is, to the essential: to conversion, to prayer, to the sacraments.”
Try thinking of it this way: like any mom, Mary knows when her children have gotten spiritually lazy. The apparitions are one way that she grabs our attention, inviting us back onto the right track.