If you're making or modifying textures, keeping a master version with layers can be very helpful. Instead of starting from scratch everytime, you can add, subtract, recolor, etc., to come up with a new texture. I don't think Poser can read the layers in textures, so a flattened JPG seems to be the best bet for rendering. If you're talking about post-render work on your images, the layers can be indispensible!