Vim: Exit (:wq) with trailing exclamation mark ! What's the purpose?

"!" means don't nag me with warnings; just do it.

If you try and vim /etc/hosts, and make changes and try and save with :wq! - the "!" is moot. That is a real error that can't be forced thus use of "!" won't work.

A useful example..

touch ~/example
chmod -w ~/example
vim ~/example

If you open a file where you have READ access only but have taken away your WRITE access before hand, that warning can be overridden by the "!" (unlike permissions error in /etc/), thus a ":wq!" in this case is handy (quicker than jumping to shell to fix).

:wq! means "write this buffer then close it, no questions asked." If you have any other buffers open, they stay open and vim doesn't exit.