My understanding is that, by default, Apple devices have an IDFA (Identifier for Advertisers) that apps can use to correlate usage to a specific device. That identifier is going to be the same regardless of app, so it can easily be shared between data aggregators. You've been able to turn this off for ages deep in your apple settings - but of course most people don't bother with that (70% is the number I found). This update brings that feature front-and-center by making it a popup. I suppose that it also makes it app-specific instead of the previous system wide setting? Not sure on that one. At least that is my understanding, if someone knows better please inform me.
