Without having evaluated each piece beforehand, you have no way of knowing whether one or more of the pieces was built incorrectly, whether your integration was faulty, or both. Imagine the amount of time you would waste trying to analyze what was wrong with the car. And since you would have confidence in the individual pieces of the car, you would have more confidence in the car as a whole. For the same reasons, it is important to unit test software. But, just as you wouldn't test the car parts without test-driving the completed car, you must also follow unit testing with integration testing and functional testing.
Unit testing is just one part of a good, solid software development process.
Unit tests are typically organized into groups or suites of tests. A project's unit test suites should always execute successfully at 100 percent. Any failure in the unit tests instantly becomes a top priority for responsible developers. These unit tests are useful for programmers long before deployment. A wide-reaching unit test suite bolsters confidence by allowing developers to change the code without fear of breaking it.