iOS (and OS X) does this: when a permission is requested, the developer can provide a usage description string to explain what that capability will be used for: https://developer.apple.com/library/ios/#documentation/Gener...

Note that on iOS permissions are requested to the user at runtime when the app tries to access a restricted capability, which helps understanding the request because it happens in the context of what the user is doing with the app.

Also, each one of them can be accepted or declined separately and turned on or off at a later time.