I think the only thing that makes Substack moderately notable is that it has become a platform for the kind of journalists who would never be allowed to write for the Official Channels(TM). Kind of like Patreon has for podcasts. Of course, there's a lot of garbage on it too.

Politico is malding because a handful of muckrakers have found a way to keep on raking muck without being smote by the institutional media. Substack itself is just trying to get their bag though. It's not like they're actually on a mission to save journalism or anything.

As far as a use case goes, the only way I could see myself using it is to follow someone I'm interested in. I couldn't care less if they're on Substack, a personal blog, or whatever.