Campaign A: Show 10,000 random people ads for espresso grinders.

Campaign B: Show 10,000 people who recently looked at espresso grinders some ads for them.

95% of the first group didn't know espresso grinders were a thing people bought. The remainder probably aren't actively shopping for one.

In the second group, some portion of the audience has already bought an espresso grinder, or decided they don't want one right now after all. But it's also guaranteed to include thousands of people actively shopping for one right now.

Which campaign do you think is going to result in more espresso grinders sold? The one that doesn't include you, or the one that does?

Your anecdote is not emblematic of the whole. Many people do not immediately buy big ticket items they look at online; but there is data that shows repeated marketing of a product works, and works especially well when the merchant already knows you were looking at a specific product.

Yes, this means they end up wasting ads on people who already purchased, but that is outweighed by the other portion of people who didnt already purchase the product.

I agree with you, but for a different reason.

I work for a company uses these targeted ADs. 100% of the ADs I see are for what my company sells. Well 98%, sometimes I see ADs for services we are already using.

It would be cool if when I am reading an article about how do X, there was ADs for goods and services that help me do it. That would at least make me think about clicking. Instead, I only see my companies product, which is worse the useless.

My company is paying to advertise to itself. And if it is happening to us, it must be happening to others.

People who purchase something online are magnitudes more likely to purchase it again than someone who hasn't made the purchase.

Criteo told in an interview that the click rate was 500% better with their tracking.