* Maintains a user database including authentication, plus all the support costs (password support is very high)
* Payment handling (no one does it for free) including keeping up with tax authorities and legal systems in much of the world
* Almost unlimited (re)downloads for users to a reasonable number of devices. Bandwidth is cheap but not free.
* Backup and restore for application data. (That functionality is available to apps without extra fees.)
* A curated walled garden including a review process. Apple keeping the dregs out and avoiding the place turning into a cesspool means users can be more confident about the apps and that halo effect helps all apps in the store.
* Mechanisms to extend your app such as IAP, and an advertising solution etc
* Access to a large user base, with reasonably fair rules that everyone has play by
* They make all the above work together
It is certainly worth more than zero percent!