"!" 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..
chmod -w ~/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).