Fusion of two or more images
In order to enrich the image (e.g., the combination of normal images with tera-hertz or with infrared images).
in order to eliminate the influence of focal length
medical and determination of disease and cancers.
in order to remove the effects of lighting
in order to enrich aerial or satellite images
in order to combine faces
The latter case, of course, has other applications such as anticipation of a child's image from the pictures of parents.