It would be to allow you to see what the apps are doing, and prevent them from doing what you don't approve of. Including apple apps.
It could be as simple as: prevent this app from using the network.
Furthermore, what if the app tells me that it needs network access to get calendar info or something along those lines - and then actually uses the network for that -, then subsequently uses the network access to siphon off my personal info? Certainly if you had the ability to "see" what an app is doing, you might be able to catch that, but that's well beyond the grasp of most users. Better to be able to inherently trust the app because it was built by a trusted 3rd party. Certainly I agree that it would be nice to be able to disable network access to any app I felt like though.
But, most people on HN don’t seem to understand Apple’s priorities. This is how it should be.
1. Apple
2. Users
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3. Developers.
iOS really doesn’t trust third party keyboards.
How do you deny an app network access and allow it to share your data across your own devices? With your partner?
Would you also like to give third party apps access to your biometric data? Your password storage?
commenting only the technical aspect, without suggesting anything is or is not a good idea, apple's operating systems could offer a service whereby apps could hand over data to the OS which it would save to icloud on their behalf, and hand back to only that app or other instances of it (as determined by the user). that, plus normal networking denied to the app would do the job nicely.