This check box seems to be a recent change (with no warning as far as I can see) that means if you make a change to a published post you can choose whether or not to notify patrons. You don’t want to send out emails every time you make a minor typo change to a post is the thinking, I believe.
A good idea on the surface, however there seems to have been an additional change to tier adjustments on a post.
I write webnovels with chapters available early on Patreon. Different tiers give you access to chapters at different times. This requires me to change the tier manually for a post when different tiers become eligible to read it.
Previously, when I changed the tier and saved it, the new tier patrons would receive an email telling them the post was available to them.
Now, I either inform no one, or everyone. People who already had access get notified again.
I feel this has been done because it was easier to code, not because it’s better for the end-user. It certainly isn’t convenient and not desirable to spam people with emails they don’t want. The inability to separate out users based on tiers seems an oversight, especially considering how integral tiers are to the concept of the site.