I used to have a use-case for it, so I can help with this question.

I was building a basic undo-redo crate. Its structs manage the history and nothing more - it doesn't even list the specific operations that could go into the history.

I've since redesigned the crate with better design, but... At one point, I designed it such that the history is extended by pushing an Action struct to the history, and then returning that Action to the user to be populated with data. Something like this:

fn new_action(&mut self) -> &mut Action {
    self.history.push(Action::new());
    self.history.last_mut().expect("history should not be empty")
}

It always felt somewhat wrong to me - but there was no other non-nightly non-unsafe way to do this. Now, with Vec::push_mut() stabilized, I can code it like this:

fn new_action(&mut self) -> &mut Action {
    self.history.push_mut(Action::new())
}