Have you ever noticed the abilitiy to bookmark topics? Subscribing is another, these are features I find almost useless as I never really need to bookmark a topic as it is usually always easy to find.
I use the subscribe to topic feature if I'm in desperate need of post exchanges. Then that way, I'll get an email if someone has replied to my topic. I haven't even tried the bookmarking one, though.
I sometimes use the subscribe topic feature on other forums. If i advertise on another forum i subscribe so i can see when there is a reply as i can't be bothered to keep checking all the topics in the hope someone replies.