[maxglute]

Maybe I'm talking out of my ass but this seems to be designed for regimented JP primary school life. Rigid central compartment for books. No need to accomodate for lunch boxes or water since you get that at school. Or you clothes/shoes for sports and after school activities.

[JohnBooty]

I think many kids carry a packed lunch (bento) but carry it separately in a box wrapped in a cloth or separate bag. I've not visited nor lived in Japan, I'm sure somebody can correct/expand.

https://en.wikipedia.org/wiki/Bento