• If QA is using a dev environment, that environment is likely changing. It is hard to control the state of the environment with multiple people working against it.
• Typically, QA environments mimic more closely production environments than do development environments. This helps ensure functionality is valid in an environment that is more production-like.
• Developers tend to have lots of tools and things running in their environment that could affect QA validation.
• You usually setup a separate QA environment, because you want to give the testers an isolated environment on which to test, so that developers and testers can work at the same time.
• This allows reporting on a common revision so developers know whether particular issues found by testers has already been corrected in the development code.

• Face the distinct possibility of releasing critical defects to customers because it’s not testing in a real-world environment.
• The test team won’t see the issues when the environment is not the same because the playing field, so to speak, is not even.
• Testing on a qa environment provides a more accurate measure of performance capacity and functional correctness
• As Web applications become more mission-critical for customers, it becomes increasingly important to test on environments that exactly mimic production because it’s production where customers use your application

 

Resources: http://stackoverflow.com/questions/2777283/why-should-qa-have-their-own-qa-environment-what-are-the-pros-and-cons

http://searchsoftwarequality.techtarget.com/tip/A-good-QA-team-needs-a-proper-software-staging-environment-for-testing