new messages are marked with a red flag.
you should only do 'mark read' when you are telling the system to make all the unread flags disappear. the usefulness of this is to keep the system from showing up unread messages when you know you don't want to read them.
say you're on the bb board. you read all the ones you care about. but then there's a thread about duke. you don't care. but if you leave it alone, everytime you go to the forum list, it would should you unread messages in the bb forum, even though you've read all the ones you care about. so then you click mark as read. now the forum list will say you've read them all. the next time the forum list says you have unread messages,you'll know that you really do have new ones to read.
s J t A r Y 8 H e A d W g K e