The problem is that you have no idea if those ads are working, not really.

Do I go to McDonalds because I saw an ad, or because it 3 minutes away and the kid wants nuggets? Do I shop at my local supermarket because they buy every third ad on YouTube, or because it's the closest? There are a ton of details that influence our daily choices, and I'm not suggesting that ads aren't one of them, but I'm starting to doubt that it's the defining one. When I look at what I buy it's: availability, cost, and specification. There's not a hold lot advertising can do about any of those three.

If we could truly measure which ads works and which doesn't the entire ad based economy would crash over night.

Ads don't have to be the primary deciding factor but it does make the rest of the process in the sales pipeline easy.

Whether we realize or not subconsciously our minds will associate the product and ad. When that association is reinforced by the salesman, a strategic placement on the shelf, or discount offers it is easier to convert it to sale.

This is an entire industry. It's profitable. It works. The ad based economy exists because they do truly measure which ads work and know exactly how much money it makes them.