Comment by mcv

This exactly. And getting content from third party sites dynamically. The classic example was having a Google Maps thing on your site where you'd show your, or even yet another party's data on a map. There was increasing amounts of data becoming available, apis opening up, governments releasing data sets. Combining all of that into something interesting, that was the real promise of web 2.0.

And then everybody started using that to add trackers and push ads.