For those unfamiliar, reader apps are essentially apps that display their own content like videos, ebooks, and so on, so think apps like Netflix, Kindle, Spotify, and so on. What Apple has done here basically lets an app tell its customers that they can manage their account and subscriptions through the app’s own website.
This means that if customers want to subscribe directly to Spotify or Netflix instead of going through the App Store, these apps can now direct users to the website directly. Prior to this, part of the App Store’s rules was that external linking for this kind of use was prohibited and apps that were found doing so could find their apps removed from the App Store.
In a way it’s actually kind of better for some developers because it allows them to bypass Apple’s 30% cut completely for in-app purchases. However, since it only applies to reader-style apps, not all developers will be able to take advantage of it, like game developers who offer microtransactions, but we suppose it’s a start.