Because I normally want to go back to the New Content page when I reach the end of a thread, and I'm invariably at the bottom of the page when I get there.
I've been using the Next Unread button to read the next unread post in a section until that entire section is read, but usually the unread posts are spread across many sections, so after reading through one section I currently have to scroll back to the top of the page, (which is sometime quite long) just so I can get back to the list of unread messages so I can choose what to read next.
-Neelix