The big money at stake in this particular change is ads promoting the installation of apps. Facebook’s current tracking allows them to show that a particular ad FB displayed resulted in spending on an app. Apple taking over this business with their own ad service is just another aspect of their efforts to capture an ever-expanding chunk of commerce that occurs on every iOS device. Keep in mind the context that Apple is banning apps for allowing users to use subscription services without paying Apple 30% of the subscription revenue.
For what it’s worth, Google is guilty of some of the same issues on Android. I hope any regulation cracking down on these abuses will be equally applied.