It sometimes refuses to send an old story to my subscribers (all of them) with a note telling me that I've sent this story to them before.

Obviously, that can't be true because I haven't sent a 2020 story to folks who subscribed in 2023.

So there's some cut off date or other criteria.

Another writer suggested that any story you've published before the date on which the first person subscribed to get your work is fair game.

The answer to the question "what good is it" is that it's still good for sending out older stories -- when it agrees to send out those stories.

When it sends them an old story out, it goes to all of your current subscribers.