Unit Testing Benefits

The goal of unit testing is to isolate each part of the program and show that the individual parts are correct. A unit test provides a strict, written contract that the piece of code must satisfy. As a result, it affords several benefits.

Facilitates change
Unit testing allows the programmer to refactor code at a later date, and make sure the module still works correctly (i.e. regression testing). The procedure is to write test cases for all functions and methods so that whenever a change causes a fault, it can be quickly identified and fixed.

Readily-available unit tests make it easy for the programmer to check whether a piece of code is still working properly. Good unit test design produces test cases that cover all paths through the unit with attention paid to loop conditions.

In continuous unit testing environments, through the inherent practice of sustained maintenance, unit tests will continue to accurately reflect the intended use of the executable and code in the face of any change. Depending upon established development practices and unit test coverage, up-to-the-second accuracy can be maintained.

Simplifies Integration
Unit testing helps to eliminate uncertainty in the units themselves and can be used in a bottom-up testing style approach. By testing the parts of a program first and then testing the sum of its parts, integration testing becomes much easier.

A heavily debated matter exists in assessing the need to perform manual integration testing. While an elaborate hierarchy of unit tests may seem to have achieved integration testing, this presents a false sense of confidence since integration testing evaluates many other objectives that can only be proven through the human factor.

Some argue that given a sufficient variety of test automation systems, integration testing by a human test group is unnecessary. Realistically, the actual need will ultimately depend upon the characteristics of the product being developed and its intended uses. Additionally, the human or manual testing will greatly depend on the availability of resources in the organization.

Documentation
Unit testing provides a sort of living documentation of the system. Developers looking to learn what functionality is provided by a unit and how to use it can look at the unit tests to gain a basic understanding of the unit API.

Unit test cases embody characteristics that are critical to the success of the unit. These characteristics can indicate appropriate/inappropriate use of a unit as well as negative behaviors that are to be trapped by the unit. A unit test case, in and of itself, documents these critical characteristics, although many software development environments do not rely solely upon code to document the product in development.

On the other hand, ordinary narrative documentation is more susceptible to drifting from the implementation of the program and will thus become outdated (e.g. design changes, feature creep, relaxed practices to keep documents up to date).