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 is more or less 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 can NEVER have that approach towards the product he/she developed.

Hi

There are so many reasons.

a. Attutude: Devs are always thinks if application satisfies the +ve conditons then their work is completed but the only the testers can think and test all the -Ve scenarios.

b. Impact: Devs always forgot the impact of a new change in the remaining part of an application only the devs could think of it and let the devs about impact of change

c. Sentiment: As per human phsycology no one is interested to found d bugs in his developed application and he cant found only the second person is able to found

d. Functionality: Devs may dont have the full idea of functionality as they are limited to some modules only the testers knows the complete functionality and then they can intimate the think to the devs

Srinivas

The way of thinking about the quality of same product might not be same from
person to person And coming to why tester only needed to do testing is becoz

1. Testers aim is to improve the quality of the application/product and it is
his duty for the sake he has to spent all of his time in testing the
application but where as developers cant spend lots of time in testing as their
duty is to write the code for the application

2. Testers while testing will consider all types of testing like Functional
GUI usability Integration positive negative acceptance performance etc.
but developers will consider only positive scenarios

3. Testers has to concentrate on each and every functionality while testing the
build but whereas developers concentrate on few functionalities that they are
working right now

4. While fixing the issues or whenever there are any changes in the
functionality developers need to consider the impacts of those fixes or
changes but they miss them or there might some hidden impacts and those
impacts are find out when testing is done by testers.

Please suggest me if i am wrong

Testers do testing from customers point of view while dovelopers do not.
Testers are not biased in testing because they do not create the code.
Testers success lies in breaking the code while developer creats the code.