I'm glad you brought this up, because now that I've tried it I've found it really is a handy tool in the new updated forum. I do recall seeing the option before, but it wasn't needed because my visits often "timed out" on me before I was finished reading all the new posts anyway, so I couldn't remember whether I had done so or not. Or else I logged off the internet and when I returned all posts showed up as "read."
I'm not sure how long unread posts stay lit up now (days? weeks? months?) but I was wasting a lot of time reopening a forum and finding I was already done with it even though the entire page was still lit up. Now I can click each forum off and wait until new posts build up again. Cool!
P.S. This is especially handy when one has a computer that sucks.