A Youtube video is worth a thousand words:
Remember the mimeograph machine? If you've actually used one as a teacher, you were born around 1960. The first year I taught, we had one of these in the faculty room; the next year it was gone -- replaced by a Xerox photocopier. Some teachers stored up whole courses of exercises, tests, quizes, exams, essay questions on these blue, carbon, reverse-printed sheets. They were carefully laid away in alphabetized folders. When the xerox machine appeared, they would just lay the flip side of the sheets down on the glass and photocopy the old blue carbons. They had no intention of redoing all that work.
This is where the network changes all that. Up until schools had networks, nothing much changed. Even schools that had computers didn't change much because each teacher's work was isolated -- stored on floppy disks carefully laid away in alphabetized folders.
With networks came shared storage space, an early form of google docs. Teachers could collaborate. They could use each other's work. And they could add, edit, refine and update that work.
All of this was possible before networks, but it took too much time. Textbooks and curriculum are exponentially easier to put together when you have a network and easily editable formats to work with. Teachers all over the world are posting lessons for free that you can incorporate.
Still I wish I had a spirit mimeograph machine in my basement!
By the way, I assert by my own authority that the video is accurate because I have personally used a mimeograph machine and what you are seeing is exactly how it works and looks.