Having the right school supplies help children to learn in multiple ways.
Teaching reading and writing would be nearly impossible without classroom supplies because students would have no way to practice their skills. Knowledge can be transferred by simply talking, but the development of skills requires practice. Having books, papers, and pencils give children a way to take what they've learned and apply it to an actual activity.
Classroom supplies also give students various ways visualizing what they're learning. Different students have different learning styles, so it's important to teach students in as many ways as possible. A student struggling with multiplication might suddenly "get it" when it's explained through the use of physical objects.
Supplies in the classroom allow students to express what they've learned, and that expression provides educators with valuable feedback. Students may pretend to understand a concept as to not be embarrassed in front of their friends, but until you see them display their knowledge can you know for sure.
The next time we're about to praise a teacher for being great, give a little credit to their incorporation of classroom supplies as well.