You have to understand how ad exchanges work (and what they are) and what intermediaries there are. Ad Exchanges handle the actual delivery of ads. They have a

You have to understand how ad exchanges work (and what they are) and what intermediaries there are. Ad Exchanges handle the actual delivery of ads. They have a massive inventory of ad slots available on a massive amount of websites and it's their job to fill those ad slots up while making as much money per ad slot as possible. Google (Doubleclick) is not the only display ad exchange [1]. There are quite a few other ones like OpenX, AppNexus, Microsoft, and so on. You can directly buy on these platforms but it's generally better to use a Demand Side Platform (DSP) because they provide a wrapper around these platforms and you can manage bidding, targeting, etc. in one place. They also offer various programatic techniques like real-time-bidding (RTB) that can end up costing you less than you'd spend directly in an exchange. Lastly, some exchanges operate in such a way that small advertisers can't utilize them. It's a human-intensive process that involves spending a lot of money, signing and committing to insertion orders, etc. Because of this stuff, most people are now using DSP's. The DSP's buy large batches of inventory from the ad exchanges and it's their job to maximize the revenue they get for their purchased inventory. As an advertisers, I can set up my campaigns in the DSP, which might involve setting targeting options, setting up remarketing pixels on my website, etc. With RTB, I set a maximum bid amount (in CPM) in my campaign and when it comes time to serve an ad, if the ad slot matches my targeting or remarketing preferences, I compete in an auction against others and the highest bidder gets the impression. In almost all cases, the ad exchanges are going to make more money (by means of providing value to end-advertiser) selling to people who are setting their own targeting preferences rather than trying to guess. Within the targeting options presented to advertisers, there is lots happening in the background by machines that are doing things humans can't. Ad tech is pretty sophisticated, even if it sometimes seems that it's not. Ad tech has created much more value for advertisers than it's lost when it isn't perfect. [1] Many people may think Doubleclick is the exchange but that might be because Doubleclick is the most common ad slot you see on a webpage. However, that ad slot is a different product, Doubleclick for Publishers (DFP). You can drop in inventory for any exchange (or even hardcode an ad in cases of a direct buy) in DFP, so it's displaying ads from other exchanges, not just Doubleclick.