Students no longer are in only one classroom we travel from room to room all day. So logically we use a bag to carry our books, binders and other various things we may need for class in a bag or backpack. it would take up more space to have a stack or books on the floor which can get wet, stepped on, and dirty then one simple bag. it takes more time to go between classes to get the binders and or books we need for our next class then to simply carry it in a bag