This is a good read. For a long time I explained to people that the value of search advertising was that it matched a buyer who had just asked a buying question with advertisers who wanted to answer that question. This was HUGELY different than previous Internet "banner" advertising. Think of it this way; I put a poster up in the metro that says "Best Deals on Rolex watches" and a million people a day "see it" but a vanishingly small number want to buy a Rolex. However, if Google gets a query "Where to buy a Rolex" or "best deals on Rolex watches", well you know that person is perhaps already considering buying a watch, so you want first crack at the deal.

The secret sauce was instantly pairing "commercial intent" (or the intention to do some commercial action, buy, sell, hire, Etc.) with vendors and firms who can satisfy that intent or "aspects" of that intent. (You might get an Uber ad on a query for "restaurants near me" for example)

Amazon has that in spades. If you are on the Amazon site, that is a huge signal that you are intending to make a commercial transaction. Either with Amazon, or perhaps you are reading reviews of things you can get locally, but the probability that you are about to do something commercial is much much higher. (So does Facebook along a different vector but that is a different post for a different time.)

I know from experience at Blekko that one way to piss off Amazon is to out advertise them :-). That just told me that they considered Ads strategic and selling stuff was secondary to that goal[1].

[1] I reasoned it this way, if selling stuff was primary then any additional exposure is "good" even if you have to pay affiliate fees for it. If advertising was the money maker, then someone doing it better than you (converting your ad revenue into their affiliate revenue) then you respond harshly to that.