“Isn’t required” is also sort of a myth. You aren’t required to buy supplies, but you are required to be an effective teacher, and it’s really difficult to do that without shelling out some extra dough.
For one thing, efficiency matters. The amount of stuff you could be doing to be a better teacher is endless: you could always grade essays more carefully (write more comments, give more examples), call more parents, write more versions of the quizzes to prevent cheating, read more materials that might be useful . . . you are never done, but there is a cap on how much you actually can do. Therefore, anything that makes you less efficient carries the opportunity cost of doing those things that make you an effective teacher: Clipboards, hanging files, legal pads, file boxes or filing cabinets, etc, aren’t required to do the job, but they free up time to be better at the job.
Other things teachers buy themselves help more directly with instruction: everything from whiteboards to overhead pens to construction paper. This is where all the copying comes in: IME the best teachers make lots of copies because those are the ones going beyond “read the book and answer the questions”: they are the ones making activities and excercises that address the needs of their specfic kids, bringing in newspaper articles that are relevant to what the class is discussing, etc.
Lastly, teachers spend money to create a feel in their room. Having pencils and pens and tissue and not being a complete hardass about them makes your room feel more inviting, makes the kids more open and ready to learn. Having posters on your walls makes the room energetic and passionate about the subject. Knowing that if they can’t afford to go on the field trip, they won’t get left behind makes kids feel comfortable.
You can do your job with what they give you, but IME people who try have cold, hard rooms, boring and repetitive classes, and never do more than the bare minimum. While those people may hang on to a job in a weak school, in a strong school they will be run off—you can’t really fire a teacher, many places, but you can make their life hell until they go. So while you aren’t really required to buy stuff to be a teacher, you are required to be a good teacher, and that takes stuff!