Master Fu had been taking one of the tests of becoming a guardian of the miraculous. Unfortunately, he had to sit for 24 hours without eating or drinking anything, guarding the miracle box with a special cane. Master Fu, or at that time, Fu was just a young boy. His temptation and hunger took over and he decided to use the butterfly miraculous to make a sentimonster to fetch him some food. That, as he expresses, was his greatest mistake. The hunger inside of him led to an opposite effect and a sentimonster was created whose main aim was to devour the miraculouses. Before he could control the sentimonster with his cane, which contained the Amuk, the monster ate it. Fu fled with the miracle box but the sentimonter chased him until he fell over the edge while Fu was saved. That resulted in something bad. It resulted in the loss of the Butterfly Miraculous and the Moth miraculous.
You can view the episode where they tell this over here:
Nothing we know would have happened as we know it if this incident wouldn’t have occurred. Master Fu would not have gone to Paris, there would be no miraculouses lost, Marinette would not get the Ladybug Miraculous and Adrien would also not get the Cat Miraculous, Marinette and Adrien will grow up and marry each other, Gabriel would not have any knowledge of the miraculouses and the miraculouses would live hidden from the world. The major and crucial thing which would occur would be that if Master Fu clears all the tests and becomes a guardian, he will not get this Miracle box! This Miracle box is the main and the most powerful of all of them and the proof is in the episode, “Furious Fu”. So we should be happy that this event has taken place.