Take for example: you have a human character for which you wanna create material map for diffuse channel,bump channel, specular channel, and so on..if you use procedurals for this, more often than not your gonna end up getting strange effects and probably not get what your looking for(i.e. precise control over the shininess of skin, say at the nose tip and some bumps at the forehead to create creases, etc..)..but if you lay your uv's well and paint the textures using these uv's then you will get the precise control of where each detail will be on your character's face(bumps at the forehead for creases for example)....