They could frame the in-app content in ads. They wouldn't be able to do that using iframes on the web. (I guess in theory if they ... exploit Chrome then they could put ads there too).

something like this https://news.ycombinator.com/item?id=45051920

It's more of a second order effect, once you train people to turn to your app for something instead of the open web you can then steer them towards more owned and operated experiences within that app instead of leaving.

Meta is the perfect example of this. In browser newsfeeds have the same ad load as the app but by steering everyone into an app and then controlling the experience there they make a shit ton more money.