My oldest, Lily, has books piled everywhere in her room. On the floors, on her bed, in her closet, on the dresser and 4 shelves on her wall. I asked myself why and when I couldn’t come up with answer I asked her. Apparently books are only meant to stand upright on shelves and not horizontal so she left the shelves empty.
All they are is a rectangle of fabric sewn along 3 sides, filled with rice and slip stitched closed. They do a great job holding up her books and have the added benefit of being perfect for throwing at her sisters when they decide to intrude on her sanctuary (and are much softer than the books she was previously using)