It simply allows you to reset posts as having been read. So, at any given time you can mark all current posts as having been read. Now the only way that happens is if you log off, and then back on.
This helps for those of us who basically leave our 'puter set at lawnsite all the time. I can mark all as "read" and even though I don't log off, when I come back and refresh, I will be able to tell any new posts.