Some people want to "watch" a thread for future posts of relation to topic at hand.
Anytime you make a post, it automatically subscribes you to that thread.
However what most don't know is there's a tab at the top of the page that says "thread tools" that when you click on it, gives you the option of subscribing to a thread w/o having to actually make a post.
It helps cut back on the clutter of posts for subscriptions only. I use it quite often.
Hope this helps.