Hi !!
They might be quite a lot of reasons
- to avoid cheating
- to avoid being stolen , to get hustled
- to stop being always on social networks during time off and try to be a little more in real life with your school friends, chat with them for real
- so to get used not always being on a screen, as a "detox"
They made an experience in England (I read that in the newspaper) and the students said they felt less tired and had better marks.....
Hope that helps !