I'm pretty sure I read here on Hacker News that one idea to advertise the exact same product that you just bought (I had that with a basketball I got for my daughter) is that they want you to feel good about the purchase. If you see it in ads after you bought it, you'll start thinking it's a popular product and will feel that your choice to buy is being validated.

And they of course get to repeat the name of the store for you so that you're more likely to think of them the next time you need a related product, even if it's not a basketball. You'll remember that you've seen the name of store X in multiple places, so they must be popular.