This brings up an interesting point. I think most advertisers consider display to be primarily about branding and not so much about conversion. This makes sense

This brings up an interesting point. I think most advertisers consider display to be primarily about branding and not so much about conversion. This makes sense because even with relatively targeted ads, you're probably not in search / buy mode when you're simply viewing a website. That happens when you search for something (insurance, tires, etc.). However, there are studies that show an increase in overall ROI when using display and paid search together. In this context, CPA ads are beautiful to the advertiser because they know they won't get conversions on them. But people are clicking. So they get free branding impressions without the cost of impressions!