The underlying secret to beautiful user interface design is realism: making 2D objects on your screen appear to sit in 3D space with volume, surface properties and undulations that might appear in real life. These faux 3D objects have highlights and shadows just like objects on your desk might have, and they have textures that emulate real objects from glass to sandpaper and everything in between. Designing beautiful user interfaces has more to do with the why than the how.
— from Crafting Subtle & Realistic User Interfaces by Mike Rundle
I agree. Materials are very important even for digital designs. More than many people think.