Smooth looking images have every pixel quite similar to the ones all around them, while still being different enough to form a picture that isn't just one solid colour.
When a shader program is used to make an image, it is typically fed with input values (say x and y coordinates) that are only slightly different for each pixel.
If those slight differences are preserved in the calculations the shader makes, the output image will be smooth.
Interesting images however need changes in them. Tools like step, mod and abs are good for creating those, but still they tend to generate quite predictable images, which are actually not so interesting.