You click "Post New Thread" and you write something but you realize it was in the wrong section of the forum. Instead of copying the text and clicking through to the right forum you would just click on a scroll-down menu and choose the right forum.
I do quite like the sound of this. Provided the option isn't too intrusive, then I'm all for it. I mean, I find that, personally, it's quite rare for me to mistakenly begin creating a thread in the wrong forum, so I wouldn't find much use for it. It's be nice to have it there, though, as long as it doesn't ruin the sleek, minimalist look of the new thread page.