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I think the idea is that people writing about their own work is tough to distinguish from self-promotion. As mentioned, it helps establish the subject is actually noteworthy enough to deserve an article, but it also helps establish it as more objective. Otherwise Wikipedia would be (more) full of crackpots creating articles about whatever fool idea they just came up with. An objective article should consider what others say about the topic.

So, an article about flat-earth theories shouldn't only use sources created by believers in these theories - it should involve published critique of those theories from others. Similarly, probably politicians shouldn't be allowed to write their own articles, etc.

Secondary sources are not "hearsay" but I'm sure you are fully aware of that.