Textures, usually seamless are used by designers to keep the whole website size small and also high resoltion. Since seamless textures can be tiled to any resolution, using big textures is unneccassary in modern designs. Also raster images will look nasty in high reosltion lcd monitors which are common now-a-days. But subtle overlay of textures from wood, rock, sand, paper can be much more applealing since only overlayed parts of those textures will be visible usually dust, noise or lines etc. Here in this article you can see 30 Examples of Subtle Texture Overlay in Websites which will inspire you.