It is true that apparitions fall under a category called "private revelation", which is not required for us to believe in as part of the Catholic Faith. So La Salette, Lourdes, Fatima, Medjugorje, etc. are all considered private revelation.
But, many incredible graces come to those who believe and follow these private revelations. We have all heard stories of dramatic healings of mind, body, and spirit in all of these major Marian apparition sites. Medjugorje is no exception, and I have personally witnessed many incredible miracles that have come about as a result of the special graces given in Medjugorje. Our Lady has said many times in Medjugorje that we are living in "A Time of Grace". Our Lady asks us to take advantage of this grace to change our lives back to lives that will lead us to heaven rather than eternal damnation. As much as modern theologians and preachers of many denominations try to white wash this reality, there are only two places we can ultimately go, heaven or hell. Satan has done a very good job at making people believe he does not exist. We must put on the "Full armor of God" as St. Paul says if we are to survive satans attacks.
It is incredible to imagine that Jesus loves us so much that He has sent His mother to earth as early as the 1300's to warn us, and implore us to change. Historically we have done a poor job of listening and responding.