Why? You visit say... kogan.com.au, they know what fridges you look at. That's intent to buy. You're cookied as #12412214. In the backend, that cookie ID is lab

Why? You visit say... kogan.com.au, they know what fridges you look at. That's intent to buy. You're cookied as #12412214. In the backend, that cookie ID is labeled as FridgeLover. The next website you go to that has an ad slot, when the ad call is made, your anonymized information is submitted, and if the advertiser wins the bid, the advertiser shows you ads of fridges, because the advertiser remembers that cookie ID 12412214 is a FridgeLover. It's anonymous (though significant efforts can be expended for less anonymity). And most importantly, it helps users recall products they've seen. It is relevant to the user (fun bits: buy retargeted ads on competitor sites!)