I went to a small independent school from 3-16 and we were always told we had to back our books to help keep them in good condition through the course of the year.
In the infants you weren't expected to back your exercise books, but then the books never left the classroom - the teacher would collect them after every lesson and put them away - and they were mostly just full of stuck in worksheets anyway so they stayed in brilliant condition all year.
In juniors, students were responsible for their own exercise books and in the upper juniors you would start to get homework that meant you had to take books home so you had to back your books to help protect them. We all had those old victorian lift top desks and I used to love opening mine and seeing a pile of books all stacked neatly sporting the latest in patterned sticky backed plastic and if you had forgotten your homework you could 'accidentally' hand in the wrong exercise book to give you some extra time
It also helped when teachers were returning exercise books after marking them as you could see where yours was in the stack, so if the "teachers helper" wasn't getting to your book quickly enough you could whip it out of the middle of the stack of books as they passed.
Same for when you got to seniors - Yr 7/8/9 would exclusively use exercise books for all subjects so they had to be backed to help protect them when you were carrying them between different classrooms/cramming them into lockers or desks/taking them home to do homework etc. I remember that by the end of the year the backing would often be worn and usually a bit grubby round round edges but the actual exercise book cover underneath was practically as good as new.
By Yr 10/11 when you were doing GCSEs you no longer got exercise books and could take notes however you wanted as homework either had to be submitted as loose leaf paper dropped into the teacher's In tray or was a typed essay anyway so exercise books weren't necessary.