I'm surprised there's so many comments here about demographic data while that's only a really basic view into current ad tech. Since the late 2000s ad networks have invested heavily in retargeting [1]. It's quite common that people end up in a vendor, add something to the cart and never finish the transaction. Retargeting attempts to catch behaviors like that and adjust to display the items that the user showed interest in. I haven't been in that industry in years, but in the 2020s I can only expect much more nuanced systems based on ML. And everything falls apart without being able to keep a history of what you do and who you are.
[1] https://en.wikipedia.org/wiki/Behavioral_retargeting