I worked on user tracking all my life. You can track user activities, interest etc, much better on an app. This data is valuable for in-house use and to sell it to the highest bidder.
yes yes, there are third party DMPs. Phone companies, offline stores etc. all share data with these DMPs. It is quite disgusting. Particularly shady practices done by FB.
But nothing can defeat first party tracking you get with the app.
Meaning collecting data even if the app is not opened? I would suspect so.