Why is testing important if the code already works?

Code that appears to work for known cases may still fail under edge conditions, future changes, or unexpected inputs. Testing provides confidence that the system behaves correctly across scenarios and helps prevent regressions.

Automated tests also improve maintainability by allowing developers to refactor safely and catch issues early in the development cycle.

In production systems, testing is less about proving the code works once and more about ensuring it continues to work as the system evolves.