Can use subtraction for image masks (e.g. to remove constant background).
Subtraction is used in angiograms: after injection of contrasting agent, subtract the original image, then do constrast stretching to emphasize the contrasting agent.
[Gonzales and Woods figure 2.28]
top-left: no contrasting agent, top-right: with contrasting agent
bottom-left: difference; bottom-right: with constrast stetching
Multiplication can be used to correct patterns of uneven illumination.
Below the left image has an uneven pattern of illumination, shown in the middle image. The right image is equal to the left image multiplied by the recipricol of the middle image.
[Gonzales and Woods figure 2.29]
Below, the left image shows "vignetting", which is a reduction in illumination with distance from the centre. This is most obvious at the corners. The right image shows the "de-vignetted" image, produced, as above, by multiplying by the recipricol of a background image.
[juzaphoto.com]
Optical vignetting occurs because light arriving from off the optical axis is blocked by elements of the lens and travels a longer path.
Pixel vignetting occurs because off-axis light arrives at an angle to the flat CCD array, causing it to be spread out more than light arriving from on-axis.
Cameras can store lens-specific vignetting masks and use them to automatically de-vignette. See below for example masks.
[lenstip.com]