A simple but important one: being able to get a notification on your phone when a new post lands in your inbox. There are lots of subtle little things around readability, scroll performance, etc. too. There are also a decent number of Podcasts on Substack now, and listening in a web player (on mobile) is a pretty sub-par experience.
Also useless, because it is very likely you cannot read the article at the moment you receive the notification, because you are busy.
I don't know what Substack is, but it seems similar to Medium, and as such an app is unnecessary. You don't need apps to display and read text, I already have a web browser. Distraction-free reading? Just make sure your pages are compatible with Reader View, or better yet, make a distraction free design in the first place.
I turn off gmail notifications. I suppose some of their readers might want to know when the Substacks they subscribe to have new content but don't want to know when they're getting new spam.