You may have an interest in an active discussion in the Discussion Forums but not be ready to participate. Perhaps someone has asked a question to which you would also like to know the answer.
If you contribute to a discussion then you will be notified by email when there is an update to that discussion, but what if you haven't contributed but still want to be notified? As part of the discussion forums feature, it is possible to Follow a discussion which will notify you by email when someone replies.