With the web sites I run, I thought it would be a service to users to display ads appropriate to the content of the page. For example, if the page was about bash programming, you might see an ad for a book on bash.
But there turned out to be NO WAY to specify a category of ads to run on a site. It was all their algorithm. Hence my sites on programming would keep running ads for "Batman Returns". How utterly pointless and boring.
I had a page on the Revolutionary War, and the affiliate ads would be for travel agencies. What would have worked would be books on military history.
I eventually gave it up.
I.e. the ad should be on-topic to the specific web page, not specific to the reader. Not only would this be much better targeted, gathering profile data on the user would not be helpful.