If I ask a question, I would like it if I got a notification if someone as commented on any answer to the question I have asked.
So I ask "How do I foo". Joe Answers "One way to Foo is to Baz." Then Ted comments on Joe's Answer "You could also bar". I would like to get a notification about Ted's comment.
The rationale being that sometimes if a question is getting a little old (I tend to watch my recent questions actively), a useful comment will be posted and I might not see it a timely fashion.