It's useful sometimes on revisiting the forum to see if there's been any new posts (individual entries in a topic) since you last looked.
The forum software keeps track of which posts you've read and which ones you haven't.
If you use the Mark forums read option, the forum software will record every post in every topic as having been read by you.
If you then use the View unread posts option, you will only see any posts that have been made since you marked everything as read. As you read those posts, the forum software will now record them also as having been read by you.
Thus on each visit to the forum, simply using View unread posts will be enough for you to see what's been happening since your previous visit.
I have found this method of monitoring the latest posts to be better than View new posts or View active topics.