Let's say you are helping a user in Tech Help and you want to know when they respond to a question you asked, without having to hang in the thread pressing refresh. I will typically use it for that.
Or if you post a thread asking for input in general, again you don't have to hang there waiting if you're the "I wanna know right away when someone responds" kinda person. Like me.
Not sure about you guys, but every thread I start or every topic I comment on I am automatically subscribed to that thread and I get notifications via email.