Because it revealed eschatologically the lot of the Church as prophesied in Revelation. It also demonstrates that Mary plays an essential role in the survival of the Church, and was the instrument of God in sparing John Paul II’s life.
I guess I should amend that. The fate of the Church has already been revealed in Scripture. The vision of the children is, more a less, a signpost of that fate, much like the star of David was a signpost for the birth of Christ. It doesn’t add to Revelation, but brings it into clearer view for our time.