Why Japanese people sleep with 布団、Japanese mattress?
The answer is simple. Japanese houses are small. If you put a bed in your room, you can't use that place anymore. But if you put the mattress on the floor, when the morning comes, you can clear the floor and put the mattress in the closet.
Furthermore, because of the humid air of the rainy season, it is easy to mold. If you use the Japanese mattress, you can hang out it easily. And to use the Japanese mattress, Tatami is important, because it is better at ventilation than flooring.