Indeed. The main reason is to quickly see that your topic has got attention. You browse the topiclist and if a blue bar is at the top, you know someone replied to your topic. If you read everything, you won't mind. But if you're as busy as me, you want to check a forum, see if your topic has a reply, check it out, return to the index, quickly scan for new topics and then mark everything as read
Thats what I do when I'm really busy. In the past I sometimes missed my own topics, so this little hack is really handy, not only for me, but for everyone.