Actually some sayings are very funny. They make sense, but in the modern western world, they are really hard to imagine, especially in the cities.
Way back, houses had thatched roofs-thick straw-piled high, with no wood underneath. It was the only place for animals to get warm, so all the cats and other small animals (mice, bugs) lived in the roof. When it rained it became slippery and sometimes the animals would slip and fall off the roof. Hence the saying.
It’s raining cats and dogs.