
Lúcia Santos (left) with her cousins Jacinta and Francisco Marto, 1917
Source: Wikipedia - Our Lady of Fátima
First a bit about 'miracles' in general.
The Catholic Church is notorious for using 'miracles' to convince its believers that it is the one true church, believing in the one true god and that it has a special relationship with the ruler and creator of the Universe. The problem it has to overcome is the problem of having no evidence whatsoever for that claim, so it needs trickery. Experience has shown that 'miracles' are particular effective on the poor, uneducated and unsophisticated people who make up the bulk of Catholic congregations worldwide.
Miracles are especially useful for the Church because, by definition, there can be no physical contradictory evidence, just as there can be no physical evidence for them, because otherwise they wouldn't be supernatural 'miracles' but perfectly natural events with perfectly natural explanations. 'Faith' fills the gap and renders the lack of evidence a mere inconvenience, even, in some strange way, confirmation.