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).