I was similarly confused by this with a Discourse instance I’ve set up to replace some mailing lists. I didn’t want to make the default behaviour be mailing list ish so I needed to provide instructions for users that want to keep a mailing list like experience. (Watching desired categories, rather than receiving everything)
The related preference above is titled with this:
Send me an email when someone messages me
So I have customised the text on our instance to change this:
Send me an email when someone quotes me, replies to my post, mentions my @username, or invites me to a topic
into this:
Send me an email when I receive any other notification
I’m not entirely convinced it’s the right wording but I do think it works well with related texts, such as when choosing to watch a category:
You will automatically watch all topics in this category. You will be notified of every new post in every topic, and a count of new replies will be shown.