It is possible they don't get 30% cuts of any of that sale of info so want it out to avoid contagion ("but they dont pay 30% on X" spreads to "I don't want to pay 30% on Y and maybe we'll go to court over it").
Based on the emails that have come out from various trials, they want a 30% cut of everything, eventually including real world services mediated through the phone.
App Store is 14 years old. If Apple wanted to take a cut of literally everything that was facilitated using the phone e.g. Amazon or eBay they would've done it a long time ago.
The benefit to Apple is the same reason that Google invests in Project Zero.
Higher consumer confidence that a product is safe = higher engagement = higher revenue.
Based on the emails that have come out from various trials, they want a 30% cut of everything, eventually including real world services mediated through the phone.