Are reviews meant to help media creators?

I don't really think so. Reviews, in my estimation, have two primary applications: to inform consumers and to analyze media. Neither of which is terribly helpful for, you know, the poor souls stuck making the stuff in the first place -- beyond maybe whatever limited utility can be gleaned from knowing what your audience wants and/or expects.

Writing a review should be fueled (only IME, of course) by three desires: wanting to talk about media you find interesting, encouraging other people to consume the same media (because, hey, maybe they'll find it interesting, too) and, lastly, working to understand just why and how it's interesting in the first place. That's the ideal.

There's also the inverse, which is typically far less pleasant: wanting to talk about something you wanted to find interesting, but didn't; or discouraging people from consuming the same media (because, hey, maybe they'll also be disappointed); or, lastly, working to understand just why and how it's not interesting in the first place.

A good review will examine thematic elements and patterns that the author may not even be consciously aware of in the first place, that they cannot readily replicate. Well, again, I'm only speaking from my own experience here, but I can at least say that I've had people read all manner of things into my short fiction that I never intended or even noticed, because that sort of thing just... flies out on its own. Understanding how other people read your work can be valuable, but IME works much better in conversation, where you can discuss everything face-to-face. Reviews, by their nature, are a bit too one-sided to be as informative or constructive. In other words, workshopping works best. (It's also a lot easier to take advice and criticism from a human being than a screen, and the absolute worst thing a writer can do is put themself in a position where they don't think they need to listen to or even solicit advice, which is a very easy headspace to find oneself in when all of that interaction is pixels.)

Which isn't to to say it's bad or wrong for the people who create media to read reviews of that media -- just that I don't think that's really why reviews are written in the first place. Reviews are, I think, written for literally everyone else.

EDIT: I also think there's a bit of a danger in misunderstanding your audience and your role. If you want to dictate how an author should write, the appropriate context is in a letter (or analogous direct message). It's kind of pretentious -- and presumptuous -- to write a review with the assumption that you're speaking to whoever created the media you're reviewing. It can also create a weird parasocial dynamic. Well, not to say that there's such a thing as a non-weird parasocial dynamic. I think it's also disrespectful to your audience, as directing criticism at the author(s) is effectively cutting the rest of the audience -- your peers -- out of the conversation.