>The site wants to over estimate clicks, the advertiser wants to under estimate

Only if you show the ads. Many companies do not, but still use lots of trackers. Why? Because their performance marketing team is trying to find the right mix of advertisement channels, so they go for paid search and clicks to Google, Meta and lots of other AdTech. In that case trackers are needed to optimize spending by analyzing user behavior. If certain cohort spends more time on the site, they will get more ads of it. If another cohort leaves the site quickly, they will see less ads. The promise of AdTech in general is that they personalize ads as much as possible to reduce your customer acquisition costs (CACs) - you won't waste money on showing ads to people who won't buy your product. So they need the data and they have to own it, because personalization is their competitive advantage.

Arh! That makes a lot more sense. Thank You. Sometimes I do wish I could learn a lot more about online advertising. But it is mostly a forbidden topic on HN.