Many C# developers question the purpose of interfaces until they hit unit testing. A recent DEV Community post explains why interfaces exist: to test business logic without touching real infrastructure. By mocking dependencies, developers can verify that a service behaves correctly without worrying about databases or repositories.

The tutorial builds a test project using xUnit and Moq to test a `HelloService`. Instead of hitting a real database, the test creates a fake `IHelloRepository`. It then checks if the service calls the repository's `Save` method exactly once.

This approach follows the standard Arrange-Act-Assert pattern, ensuring tests are fast, predictable, and isolated. Separating logic from infrastructure is foundational to modern DevOps and CI/CD pipelines. When tests run quickly and don't rely on external systems, teams can integrate code confidently.

Interfaces act as the contract that makes this isolation possible. This lesson transforms interfaces from an academic abstraction into a critical tool for building reliable software.