1.we need testers because developers are not good testers.
2.Misunderstanding will not be detected ,because the checker will assume that what the other individual heard from him was incorrect.
3.Improper use of the development process may not be detected because the individual may not understand the process.
4.The individual may be blinded into accepting erroneous system specifications and  coding because he falls into the same trap during testing that led to the introduction of the defect in the first place.
5. Information services people are optimistic in their ability to do defect free work and thus sometimes underestimate the need for extensive ttesting.
6.Without formal division between development and test, an individual may be tempted to improve the system structure and documentation, rather than allocate that time and effort to the test.

1. The primary reason isĀ more or lessĀ a psychological one.
Developers due to human tendency are not able to figure out the issues in the code which they themselves have developeed. On other hand a Tester is a second person who looks into the product from a different point of view.

2. Testers are considered "constructive destroyers". The destroy the product inorder to construct it in a better way.

3. Everytime developers will not get time to test the whole product along with their packed development schedule. Testers are sitting and waiting for the product to come on their desk.

4. Tester will have a breaking approach. A developer canĀ NEVER have that approach towards the product he/she developed.