When you build an app for iOS, you are building for one of the most advanced mobile platforms with the best native experience for the user.
Your app, whichever Apple device it runs on, has to follow the Human Interface Guidelines or HIG from Apple. There are a number of reasons for why your designers should design following these guidelines, which can be found here at https://developer.apple.com/ios/human-interface-guidelines/. You can also find a HIG for tvOS, watchOS and macOS.
iOS users have been using their devices for many years now. As more business systems are being mobilized, users can use their devices at work as well. These users have the same high consumer level of expectations for user experience, when it comes to the devices, apps and tools they use at work.
Following these guidelines will ensure that your app looks like, feels like and responds like an app that users expect. Following design guidelines makes sure that your app will feel familiar to the user, working the same way as others do, with respect to interaction, controls and design, hence reducing user problems. If you build an app that responds or interacts in unfamiliar ways users will probably respond negatively and/or will need training on the new functionality you have taken so much care to implement.
SAP Fiori for iOS is built upon Apple’s principles, providing a consistent and holistic user experience for SAP software and can be found at https://experience.sap.com/Fiori-design-ios . Here is where you can get to know the basic principles, explore the different UI elements, views and app types.
We should not forget the access and advantages we as developers get, to all the features and functionality of the awesome Apple hardware and software combined, when building native apps based on Swift and the iOS SDK frameworks.