You might use this to call some custom scene generation utility before each frame. This utility could rewrite your .pov or .inc files if needed. Perhaps you will want to generate new .gif or .tga files for image maps or height fields on each frame. They cannot be used from the command line. They may only be used from INI files.
Most operating systems allow application programs to return an error code if something goes wrong.
POV-Ray itself returns such codes. We would probably want POV-Ray to abort as well. Sometimes a non-zero code from the external process is a good thing. Suppose you want to test if a frame has already been rendered. You could use the S action to skip this frame if the file is already rendered.