Unless something has changed, this automatic digest is sent to a user only when they don’t log in for a week (or the time they configure in their preferences), and it is customized for each user, based on their activity and preferences.
I mean, I love these digests, they are especially useful for communities that you don’t follow on a daily basis. But in the way the question is posed, it gives the impression that if you opt in, you will get the automatic digest every week.