I’m contemplating trying one of the LLMs to help write documentation for this. Just to find out if it could be any good in providing a structure that allows new users to understand. For me this is hard, because it is all so “obvious” to me 
I do think especially in this case the user documentation could benefit from providing additional examples that the user could extrapolate from. At least that’s how I learn, but everybody is different.
OK, back to fallbacks. The idea here is that many of the objects you can interact with via shortcuts have several different defined effects a shortcut can trigger. If you had to separately set up shortcuts for each of them, that would be a lot of work and it would be a challenge to keep consistency. But if you don’t set them up for each one, it is hard to remember what works where so you’ll probably not use them at all.
For example:
If you have a shortcut set up for a slider (or module), you can also set up a separate shortcut to reset it to default value(s). @mino above has done this. Or for color picker buttons associated with the slider.
Fallbacks does this automatically.
For example, you link a key press to a slider.
Now when you press and release the key, you get the popup for that slider (to type a value).
If you hold the key and use the mouse scroll wheel, you change the value directly. If you add the CTRL key, you fine tune (changes 10 times smaller). Adding SHIFT makes changes 10 times larger. This is consistent throughout the user interface.
You can also hold the key and move mouse left-right for fine moves or up-down for large changes.
Click mouse button (while holding key) to press the “quad” (usually color picker) button. Double click for reset (also for modules).
For module shortcuts, right click is presets menu. etc etc. You can see all the defined fallbacks in the shortcuts preference tab or dialog.
If you keep holding the key (it may have been double or triple pressed, so not having to repeat can be handy) you can do multiple actions. Like first double click and then horizontal move to fine-tune starting from default.
There may be multiple layers to fallbacks. key+double right-click gives you the module instance menu, but if instead you hold the mouse after the double click and move it up/down you immediately change the module’s order in the pipe. Or key+single right click+move to cycle through the presets.
It is somewhat powerful so easy to shoot yourself in the foot. That’s why it is disabled by default.