This is a checkbox, rather than a radio button. I assume you mustCorrect. It modifies "Following Only" - it isn't meaningful if you've
selected "All Messages" (which Mark wrote as "All Email").
I had thought that users would be able to say, just send me the firstIt does do that if you aren't following any threads or hashtags (yet).
But if you are you get those too. I don't think there's a use for
getting First Messages in each thread but not the messages in threads or
hashtags you've chosen to follow.
Also “follow replies” - not sure the circumstances for this one.Only if you start or are the first one to reply to absolutely every
thread (in some groups, maybe here, I'm probably thought to be "that
guy"). But I'd normally select "All Messages" (aka "All Email") in most
of my groups anyway.
The use case is for someone who likes to use the "Followed Threads" list
on the web, or the "Following Only" option by email. With this option
checked they don't have to manually Follow threads they start or reply
to. It is more likely, I suspect, that there is no actual use case for
_not_ having it checked.