The ad companies wanted to aggregate information across multiple channels.

The example about "show more car ads to someone who watched other car ads"? It's not about showing car ad on a site whose content is about cars (or where the site owner decided they like that kind of thing).

It's about knowing you have wandered over to car comparison site recently so they can show you car advertisements when you look up sports news, show car-related merchandise when you're browsing some shopping site, show you insurance ads, etc.