Their "hidden feature" is massively simplifying the setup of a blog for someone nontechnical who wants

(1) A modern "minimalist" themed blog focused on written content (2) An email newsletter with the same content (3) Optionally allow paid subscribers

I've tried everything in the blog world, and most are a headache to maintain. The closest would be Ghost, but they haven't done as well on the GTM side. Substack has set itself apart by landing "tech twitter" influencers who philosophize... which is probably why you're seeing it bleed into the HN world.