Why Testing Is Essential

First of all; testing saves money. Some project managers seems to think that a separate test team will bring extra cost to the project, and therefore leave it out, but it doesn't take much thinking to see that a well organised test team which knows what it's doing can save quite a lot of money in the long run;

Early detection of defects, either by inspection of criteria spesification documents, or by code analysis, or even at a later stage in the testing lifecycle, can save tenfolds as much money as if the same defects were to be detected by a user when the system has been set in production.

Also, a separate test team can be of immense assistanse to the business users making the criteria, as well as to the development team creating the design and building the code, at all stages during a project. The test team can provide an external point of view, which ensures that nothing is left alone just because someone doesn't dare to speak up.

The test team can also provide a history and analysis of defects reported, which again will help the developers understand which areas of the program is more likely to contain further defects.

Also, by creating good testcases, maintenance tests can be delivered to production, which will be very helpfull to ensure that everything still works when new features are added to the software

As stated by Geoffrey Bessin at IBM: Quality is the tangible and intangible aggregate of functionality, usability, reliability, performance, scalability, supportability, security, and any other factors important to your customers and your business.

Sources;
Test Management Approach - Why Test
The Business value of software Quality
Requirement Analysis - Wiki
Inside the Software Testing Quagmire
Best practices for running your testing organisation