Just because something is obvious once shown to you doesn't mean it was immediately present in your mind beforehand, so being made aware of it can still be valuable.
But agree that there's diminishing returns from these insights and focusing more on the specific details of your life is usually more impactful.
The natural corollary of that is that you probably already have the necessary wisdom in your head (because it's obvious). And thus, if you're debating between spending two hours reading a self-help book and spending those two hours doing basically anything else, the latter is likely the better choice because there is at least some chance than in those two hours you will apply some wisdom or otherwise enrich your life.
The reason self-help books are bad is not because they contain bad information. It's because reading them burns time that could otherwise be better spent.