The plagues served to change the heart of the pharoah, so that he would let Moses’ people go. Ultimately, of course, since God could have achieved this through other means, the purpose was to strengthen the faith of the Israelites.
The miracle at Fatima served to encourage hundreds of millions of Catholics to pray for the conversion of Russia, to resist the heresy of universalism, and to know God was at work even during the horrors of WWII and the Soviet dominance of Eastern Europe. In other words, to strengthen the faith of the faithful Christians in the Catholic Church.
and/or
in a certain caricature of Mary . . .
usable later to manipulate the masses even more skillfully, seductively and . . . destructively-to-their-souls.
How would a 'vision' of the sun dancing and crashing to the earth do that?