I'm sure it's one of the most useful signals for the advertisers, but it is also one of the most irritating behaviours for the consumers, because even if this is a very high correlation in advertising terms, it's still the majority of the time not actually accurate for the consumer (like with all ads: I buy some orders of magnitudes fewer products than I see ads, even with adblocking, so even if I only bought things I had seen ads for, the vast majority are not helpful to me).

> the correlation between $just_bought_thing and $will_buy_another is very, very high

This looks like an argument against advertising the same thing. Why would you advertise something, they were already going to buy?

But showing an ad before their next purchase definitely would improve the numbers on the "our conversion rate chart" _advertised_ to ad buyers. A con played by ad companies to sell more ads.