When I used one we didn't have a designer at all, much less neatly-organized mockups.

But even if we had, we might not have discovered things like "oh, if you have the modal set to show a close button in the corner and also give it a really long title and make the window narrow then the button gets awkwardly pushed to the next line". The kind of stuff that mockups often don't cover, but real-world users might encounter some day.