IMPORTANT: Modern UEFI systems mount the firmware under the
/sys directory and make it available to the OS. DO NOT run this command on a modern system since it will remove this firmware, essentially bricking your machine.
The simplest scenario I can think of is someone wanting to delete all the data from their drive. There can be perfectly legitimate reasons to do this and the simplest way I can think of is
rm -rf --no-preserve-root /
Turns out this one is actually given as an example in
Do not treat `/' specially when removing recursively. This option
is not recommended unless you really want to remove all the files
on your computer.
Another perfectly good reason is that you want to delete a mounted file system that you've
chroot-ed into. In that case,
rm -rf --no-preserve-root / will delete the system in the
chroot environment but will leave yours intact.
I am sure there are more possible reasons, but in general it seems a very reasonable approach that my system allows me to do whatever I want with it. It's my job to be careful, the system should only enable me to do what it is that I want done. If what I want is stupid, that's my problem and not the OS's.
Anyway, this is a relatively new restriction, it was added in the 7th version of the POSIX specification (the previous one is here), before that
rm -rf / was a perfectly valid command. On a historical note, the
.. directories have always been protected from
rm, ever since 1979, when
rm first acquired the ability to delete directories. More on that here.