Arithmetic can be very useful. You can take two images and make another based on math between them. So for instance in the image below is the color texture and light map for some terrain. By multiplying them then the result is the ole shaded relief.

By using masks you can mask in stuff too. Although PSP mas masks as a feature I think its easier to use arithmetic. By using a combo of lightest, darkest, multiply and add you can mask in stuff or create extra effects over and above those in the effects menu.