Writing unit tests while developing an application is important for several reasons:

Catching bugs early: Unit tests help catch bugs and errors early in the development cycle, which makes it easier and cheaper to fix them. This means you can catch issues before they become major problems, and ensures the app is more stable and reliable.

Improving code quality: Writing unit tests encourages developers to write better-quality code. This is because unit tests require code to be broken down into smaller, more manageable components that are easier to test. This can lead to more modular and reusable code, which can make the app more maintainable and scalable.

Reducing regression issues: Mobile app development can involve frequent changes, which can introduce new bugs and issues. By having unit tests in place, developers can make changes with greater confidence, knowing that they won't unintentionally break existing functionality.

Facilitating collaboration: Unit tests can make it easier for multiple developers to work on the same codebase, as they can run tests to verify that their changes haven't broken any existing functionality. This can help to reduce conflicts and ensure that changes are integrated smoothly.

Writing unit tests is an important part of developing a high-quality, reliable mobile app that is easy to maintain and scale over time.