I agree that Substack needs a better content/writer discovery mechanism, like having categories or tags per article/post so that readers can find related articles by subject/genre/type to the one they enjoyed reading.

Unfortunately, Substack wants to do an internal reference or "recommendation" system that basically mimics a closed clique. Those that know other writers may get a friend to "recommend" their newsletter, but it will very likely not be anything that the readers of this friend are interested in since the recommendation may have nothing to do with the content, or the merits of the recommended writer, it's just who they know.

I think people are interested in the content, in a subject, or a genre/type of writing, like humor, advice, reviews, etc. They may want to spend more time with a subject rather than being given irrelevant recommendations based on who the writer reads.