When the analytics produce an obviously wrong conclusion (such as saying you should be shown more car ads immediately after completing the purchase of a car), everyone involved has a vested interest in believing that the analytics must be right in some fashion. Doing otherwise would mean taking on a big risk, straying from the herd with a different advertising strategy that's guaranteed to take the blame for any drop in sales in the near future.
Sorry to say: Probably another wrong intuition.
My husband was in an accident that totaled his car not too long ago. So we bought a new one. Then after having it for a few weeks, I realized that I liked many of the new features it had so I started looking for one, too. True story.
The first purchase of a car in our family caused (and, more importantly for this discussion, was predictive of) the purchase of a second car. I’m sure this does not only happen after accidents.
I have bought multiple cars in my lifetime. But I have *never* bought one right after another. This is not intuition, it's a fact.
No one is suggesting this can't happen --- but is it likely? Is it the norm?
"Personalized" ads assume it is --- and erroneously so in my case.
Start looking at cars online and you'll be bombarded with car ads for months after you've made a purchase. That ad money was just wasted on me --- and I'm sure on many others as well.
"Context sensitive" ads make no such erroneous assumption --- and they still have you covered in any case. You are shown ads only as long as you continue to express an interest in the subject.
You will find that the most probable (the mode of the distribution) thing to buy in transaction n+1 is either a substitute for, a complement of, or identical to transaction n.
i think no one would disagree that there are things they buy very frequently, almost on a schedule. (then again, with these items people are probably very accustomed to buying the same brand of milk every week, so ads don't seem reasonable here as well).
Ads work at human psychology; they're not fully logical. Though I'm sure there's inefficiency in marketing but if post-purchase ads weren't ROI positive, I doubt the market would be paying for them.
In a sibling comment I replied to someone with an objection that it doesn’t apply to cars… with a personal anecdote that it does, at least sometimes, apply to even cars.
Even governments do it: Having previous purchased a nuclear submarine is highly predictive of future nuclear submarine purchases. Likewise, you probably have not purchased a nuclear submarine, which makes me think you probably won’t buy one this year. ;-)
(About the only time I'm in the market for "another one of those" is if the first one was so low quality that I returned instead of putting up with it - or if I sampled a few things to see which one was good and then need enough more to finish the project.)
Not claiming that the ads don't have some influence, but they pretty much can't result in another sale...