These modes are intended for viewing stereo panoramas.
If you change the display mode (Full screen, Pageflip, Open next image etc), SPM stays in "Fit the Image Height/Width to the Screen" mode until the image is zoomed.
Once the image-height or width fits the window, you may use the (I) key (horizontal panoramas) or Shift+I key (vertical panoramas) with all display modes to autoscroll the image.
Fit height to the screen is intended for horizontal panoramas, fit width is intended for the far less common vertical panoramas.
Vertical panoramas are sometimes used for planetary images.
An example is here at Mars Unearthed
To view the vertical anaglyph-panorama, select 'File/Open Single Image' and press the following keys :-
Shift+Q (fit width to screen)
Stereo 360 degree anaglyph panoramas may be displayed on a webpage using a freely available script.
If the panorama source images need to be cropped, see Crop/Cropping Large Images.