Test-First Development — Core Benefit

Writing the test first implicitly defines both an interface and a specification of behaviour for the functionality being developed — problems of requirements and interface misunderstandings are reduced, and test-lag is avoided

Explanation

Writing tests first implicitly defines both an interface and a specification of behaviour for the functionality being developed — problems of requirements and interface misunderstandings are reduced. It also avoids test-lag, where the developer works faster than the tester, implementation gets further ahead of testing, and there is a tendency to skip tests.