Allow to draft a new thread so it can be posted later. It's been several times when it has been the case that I when writing a thread, all I want to do is save it to post it later.
Be able to close the browser, reboot the computer and then come back and find your draft and continue working on it.
This is specially needed when writing information or tutorials, or the like type of posts that are long and require thinking.
Also, some times create a thread, and decide I shouldn't actually post it - because something something has happened or happening, and want to wait until that is over to finish it and finally post it without losing the writing.
Also needed when there is an event or the like, that won't happen until later, and you want to prepare your thread to post it later, you can't unless you paste what you want to say some where else, ( it is my own habit to use notepad and write everything there, then paste it later), but should perhaps be built it?.