It's just about the ROI. Turns out people who have just purchased something are far more likely to purchase that thing again than a random person. This makes

It's just about the ROI. Turns out people who have just purchased something are far more likely to purchase that thing again than a random person. This makes them the perfect person to advertise to. Even if the percentage is quite low this can be better than advertising to the general population. Say the numbers were 1 in 10 000 impressions leads to a toilet seat sale, but 1 in 1000 people who have just bought a toilet seat buy another one soon after (first one was broken/distributor/toilet seat collector/remodelling whole home/etc). If those were the numbers it would be silly to not advertise to recent buyers. Essentially, the self selection of 'I just bought <x>' very often puts you in a category that is far more likely to buy <x> again than almost anyone else.