Why should we maintain separate testing team, is there any special reason?

NOTE : [This question was asked by rameshkota]

The main purpose a separate Testing team is required is that the team works with the aim of finding out as many Bugs as possible without being too emotional with the developer who has built it.
If a development team who have built the product, itself test the product, chances are many bugs are neglected or missed out. A person cannot find defects in his own creation. That's a general human psychology.

Developers always think creatively but tester have to think destructively to break software to find bugs.

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Brijesh Jain

Because developers follow a constructive approach to make the s/w application and testers have to work in destructive approach to break the s/w application (not literally ).

Because developers can’t contribute in different permutations and combinations of input to the system.

Because developers hunt for logic in development and programming more then possible bugs.

Because developers tend to be optimistic while developing s/w that right and correct input will be given to the s/w.

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Lack of WILL POWER has caused more failure than
lack of INTELLIGENCE or ABILITY.

-sutnarcha-

Programmers/Developers usually think of the technical issues (logic) to make the functionality work....

But test engineer generally search for the bugs in the functionality, and ensure that the functionality meets expectations as stated in the specifications.