I don't understand what point they're making either.
All you need to be is "professional". That is, before you edit a file, copy it (backup).
That way if you make a lot of (bad) changes and you want to see what it was like before you started, no problem.
Also, never delete any file that you're not sure about. Yes, this is where
mv comes in (as they suggest).
I think that I need to delete this configuration file..............
mv existing existing.old
Whoops......shouldn't have deleted that? No problem,
mv it back.
NEVER delete anything you're not sure about.
If the system crashes whilst you're editing a file, no problem, you've got an original copy.
Just be professional and you won't go wrong.