Extreme Programming methodology can illustrate this concept. XP Stories are commitments to a future conversation between the customer and developers. Functional tests are the output of this conversation. Stories without functional tests can't be built very well. Functional tests fill in the gap left by unit tests and give the team even more confidence in the code.
Unit tests miss many bugs.
They may give you all the code coverage you need, but they might not give you all the system coverage you need. The functional tests will expose problems that your unit tests are missing. A maintained, automated suite of functional tests might not catch everything either, but it will catch more than the best suite of unit tests can catch alone. Unit tests Unit tests are written from a programmer's perspective. They ensure that a particular method of a class successfully performs a set of specific tasks.