My impression is that every cert seller markets as though the world believed "You dont know it, unless you're certified in it."

My impression is that the world actually believes "You can be certified in something without knowing it."

So at best, from an applicant / employee's perspective, certificates are a slightly stronger suggestion that you might know the thing you claim to know.

Which is a useless reinforcement for something for which you already have demonstrated experience via work history.

But might be useful for something for which you do not.