Directly driving a user's device (or a device hooked up to a user's account at least) means an AI can do any task that a user can do, tearing down walled gardens. No more "my car doesn't allow programmatic access so I can't heat it up in the morning without opening the app."
Suddenly telling an agent "if it is below 50 outside preheat my car so it is warm when I leave at 8am" becomes a simple to solve problem.