Usually they're the worst, and like the expenses associated with billing and hierocracy in the medical field, everyone would be better off with that bureaucratic effort not spent.
I'd say at best, they can inform you of something you need but didn't know existed, but even your "best" case is a valid use of advertising. And, yeah, a lot of advertising is noise. That's why advertisers are so interested in targeted advertising.
Advertisers don’t sell ads, they buy them.
Ask the company who uses lots of nails whether hammers work.
I mean maybe the purchaser employee at the company has some conflict of interest around the question -- if they're specialized in evaluating and negotiating hammer purchases they'll not want the company to switch to screws.
But the company as a whole seems to have no vested interest in promoting hammers. They presumably want to sell the stuff they make with hammers and may lie to you about how awesome those products are. But lie about hammers? Why?
Just because someone believes it works doesn't mean it does.
Ad companies sell ads.
Advertisers purchase ads.
Users see ads.
That’s it.
Let me see if I can punch through your reading comprehension issues.
Coca-Cola spent over 4 billion dollars in advertising in 2019 [1], I guess if you asked them if they believe in advertisement they will tell you that they do.
Coca-Cola doesn't rely on you *believing* that advertisement work, they *believe* advertisement works.
I do not know if advertisement works personally, I am guessing that Coca-Cola believes it does and that they have reason to believe so.
[1] https://www.statista.com/statistics/286526/coca-cola-advertising-spending-worldwide/