There was a controversy a while back about how Target figured out a teenage girl was pregnant before her dad did (based on her purchases) and started sending her ads for baby products. The reason why they did this is because people don't shop around for the best deals on everyday items, not really. People think they're getting the best deal, but they're actually trapped in their habits.
One of the few times when these habits are reshaped is when a baby is born, because newfound parents suddenly start buying a lot of new stuff, but don't have a lot of free time to research the options. By detecting customers who will become parents in the near future and sending them specially targeted advertising/discounts ahead of time for new products that they'll need, you can snag them as customers for years to come.