Actually, this bag can't be used to hold things. This is not a bag, but a bow, but its shape is like a square. The bag behind the kimono is actually just to make the bow look better. After all, every place has its own customs and traditions, and not all bows look the same. Belts and bows can highlight the curve and beauty of women's figure, and belts and bows should be tied tightly to better outline the waist contour.
Moreover, this kind of hard bow is very close to women's back, which can make women walk straighter. The bow bag is very close to the back, so that when women walk, the bow will get stuck in the back, so that women can't hunch over and walk better.
In fact, this tradition originated from western missionaries. When they wear robes, they will tie a belt around their waist, which is imitated by Japanese women and evolved into this appearance.