The function keys were no more difficult to use than any other key shortcut. However since keyboards had that space they put the chart there and so could find the less commonly used commands. Everyone quickly learned which key was save, but there were many many others that they didn't use and so they didn't know - but if they wanted it they could look at the chart and find it.
GUIs are more discoverable (when done well), but DOS didn't really have a GUI option, so this was a second best. VI and emacs users sometimes print shortcut charts as well.
GUIs are more discoverable (when done well), but DOS didn't really have a GUI option, so this was a second best. VI and emacs users sometimes print shortcut charts as well.