All the dev tool companies run ads! CircleCI runs them, but the dozen or so other dev tool companies where I know the founders or early hires also run ads. You would think given comments like this on HN that they would be useless, but we all run the numbers. The numbers say that not only are they effective, but that usually we should run more of them.
It's like when people here say "oh I hate talking to salespeople". Yet, salespeople make the world go around.
"Everyone is doing it, must be good."
I believe this summarizes the ad industry quite well: https://www.youtube.com/watch?v=TysKyHXXtYQ
I guess you could rephrase your comment as this: People who run ads believe ads works.
As an outsider that is not very convincing.
Even then, I think you are being too generous. The stricter criterion is something like: "People who run more ads believe they will personally benefit from running more ads."
Ideally (and occasionally) this means that ad buyer believes the company as a whole will benefit from the advertising. But even if this belief is real, if the influence or income of the decision maker correlates positively with increasing spending on ads (and decreases with cutbacks) it can be difficult to distinguish belief from reality.
That's the problem, though. You can tell me that the user clicked on the ad and then bought my product; but was the user going to scroll down to my organic search result, click on that and buy anyhow? The advertising sellers claim that they should be credited with the sale, when all they are providing is moving the last link up; at least in the industry I was in, a user, generally speaking, isn't going to sign up with a VPS provider they haven't heard of. Sure, sure, putting a special in front of them can move you from second or third place to first place, and there's value in that, but really some brand advertising needs to be done before you can sell at all, and as you pointed out, measuring the effectiveness of brand advertising is super hard.
I suppose there are some products where the user really is going to buy the product first linked; but I think there are a lot of products where the user doesn't make a buying decision for some time, and the last click... may not have a lot to do with what order the links are in.
I think this is the real problem with online advertising; tracking that last click into a purchase is fairly easy (though, as you point out, some people don't even do that) but figuring out how much of that purchase was because the link was first, and how much of that purchase was because the user had previous knowledge of you? that's hard - I understand that a lot of research is being expended in this direction, and that this is the value proposition that Facebook claims, but... it's a hard problem, and depending on the product, you could reasonably argue that the state of the art isn't as good as the people selling advertising say it is.
That's the problem I've seen in the digital ad space -- lift is either hard to measure, or ignored completely. Causality to the sale is assumed on the ad.
On the other hand, there is a not-insignificant number of bot networks and shady clickfarm sites out there. Though, you can usually avoid being taken in by them by not being a cheapskate - quality traffic is more expensive. If you're bidding <$1 CPM for most things, enjoy funding a click farm.