I guess you've simplified the scenario to the point of it looking useless, what are you actually trying to do?
It was a simple example, yes. It is kind of complicated to explain. But the "f" key (just an example for a letter) remapping would not happen on the default layer. Kinda reminds me of the Simpsons episode where Homer is lacking keys for his typewriter, it quickly becomes funny
But from the other layer, not default, my point would be to jump to the default layer (where f types f) then press a Function key. This mechanism could be used in for example emacs, where the Function key could drop you into a modal mode and layer. So it is way to integrate the keyboard layers with a software's layer.
It could also be used as a signal to the system, so it can provide the end user with a visual marker/notication of some kind of what layer the user is at. You jump to a layer, and a function key is pressed, which the system looks for.
Can also be used for prefix keys, say you are on layer 1, and bind f to Switch to layer 0, then type ctrl-o, then programs like emacs will see that as a prefix, and the next key you press with determine the command (C-o x).