I don’t use this minor mode very often, but in rare cases, I find it valuable. When you have your frame split with multiple windows, sometimes it’s nice to scroll all the windows in unison. Try running
M-x scroll-all-mode. Afterwards, scrolling commands entered in one window apply to all visibile windows. This works especially well when you have two windows split horizontally (
C-x 3, side by side). To turn the behavior off, just run
M-x scroll-all-mode again.
P.S. If it weren’t for Ediff, I’d probably scroll-all more often.