You buy x at y store, y store has an ad buy with ShadyAdCo. If you see product x in an ad within z time afterwards, it counts as an influence towards your purchase. ShadyAdCo just made money, and y store can give record numbers.
EDIT: typo
It also fails to account for awareness being a major reason to buy ads. Ford want to advertise to everybody (probably including Amish) because most people will eventually buy a car, and it is important that you think Ford then. Even if you bought a car yesterday it is important to them that you not forget that you can buy a Ford - they know you won't buy for a few years, but they still want you to think of them.
"This location tracking ability has allowed Google to send reports to retailers telling them, for example, whether people who saw an ad for a lawn mower later visited or passed by a Home Depot. The location-tracking program has grown since it was first launched with only a handful of retailers. Home Depot, Express, Nissan, and Sephora have participated."
[1]https://www.washingtonpost.com/news/the-switch/wp/2017/05/23...
Ask yourself why Facebook counts "post-view" conversions instead of only post-click like AdWords and most others.
New client had been working off the assumption they were seeing ROAS of around 3 (not terrible). When we showed them how to see just the post-click, ROAS is now at 1. That means even if they are selling a product they received for free or stole, ie COGS = 0, they still only breakeven.
Attribution is the key. All this assumes that ad companies and Facebook are honest in the first place about the number of views and clicks an ad has received. Their behavior does not engender much good will for a benefit of the doubt situation.