This is beacuse in order to not to while away time playing al the time and get far away from book
tiime is precious somthing can be learnt new during holiday
so they give in holidays
even if they dont we feel sometimes boring catch some book

the first reason is that after school reopens many of the students will have no idea, they will be blank.In subjects like maths you will find hard to do sums so to keep study in touch and practice it teachers give projects and you will also have more time to do projects so they give it in leave days