[thrwy_ywrht]

>If someone clicks your ad and buys the product during that session, you know the ad worked

This is not as clear as you make it seem. Let's say there's a hypothetical product that a consumer only buys once every 5 years. If someone clicks your ad and then immediately buys the product... what if they would have bought it anyway, tomorrow, or 5 minutes from now, without the ad? How can you test that counterfactual?

If I buy cat litter online once a month every month for 5 years by going to example.org/catlitter -- but then they decide to start advertising on facebook, so now I click the facebook ad once a month to buy cat litter from the same site, are the ads "working"?

[tetsusaiga]

The idea is because it's happening at scale. If you're getting hundreds or thousands of customers per day, the likelihood of this happening gets lower by some statistical proportion, especially if you don't have the organic exposure.

Also, definition of working: You make more than you spend, and if you stop the ads, you stop making as much.

That said, I'm not arguing for 100% accuracy either. It's certainly not. Simply that it's possible to be more accurate than not, which leads to profitability.

One more also- a comment above about how I failed to mention I'm not talking about paid search ads so much as other types like FB, YouTube, banners, etc.