Need for software testing

Testing is a major component of software development, and is a major science in itself. What is the need for testing ?
Software development involves developing software against a set of requirements (whether they be a specific client’s requirements for a project, or generic requirements for a shrink-wrapped product). Software testing is needed to verify and validate that the software that has been built has been built to meet these specifications. Testing ensures that what you get in the end is what you wanted to build. Testing enhances the integrity of a system by detecting deviations in design and errors in the system. Testing aims at detecting error-prone areas. This helps in the prevention of errors in a system. Testing also adds value to the product by conforming to the user requirements.
Another way to say this is: The desired result of testing is a level of confidence in the software so that the organization is confident that the software has an acceptable defect rate. What constitutes an acceptable defect rate depends on the nature of the software.
Another way of saying this in an even more business like manner is: Testing is a process of technical investigation, performed on behalf of stakeholders, that is intended to reveal quality-related information about the product with respect to the context in which it is intended to operate. This includes, but is not limited to, the process of executing a program or application with the intent of finding errors.
And what are some the common things that necessitate the need for a testing plan:
1. Communication gaps between the requirement specifier and the developer
2. Over committment by the developer
3. Not proper understanding of all the complexities of the system
4. Inadequate requirements gathering