Mobile app development.  You need someone with proven expertise in the chosen programming language since it directly influences the code quality.
 Collaboration. Good engineers should collaborate with Product Managers, designers, QA, and DevOps specialists to ensure a smooth and efficient development process.
 Testing and bug fixing. Apart from the code writing, mobile app developers perform unit testing to verify the developed features work as expected.
 App publishing & distribution. A mobile app developer should be familiar with the App Store and Google Play guidelines.