The policy no longer applies but for a long time, from the start of the app store, you weren't allowed to make apps that "duplicated the functionality" of the built in apps.
Notably Google made their own maps app and Apple banned it:
http://www.businessinsider.com/apple-screws-google-over-lati...
But it also applied to iTunes competitors:
https://www.engadget.com/2008/09/12/app-disqualified-from-ap...