Can you explain the concept of 'screentones' in manga artwork and their purpose?

Screentones are sheets of transparent plastic with pre-printed patterns of dots, lines, or textures. In traditional manga art, they are applied to the inked artwork by cutting and adhering them to create shading, texture, and atmospheric effects. Digitally, these patterns are replicated using digital brushes and layers. Their purpose is to add depth, contrast, and visual interest without the use of color, a crucial element given manga's often monochrome format.