Why do top IT companies with six sigma certification need CMMI certification???? how will it help them? In case they refuse this certification will it affect its market reputation?
No company "needs' either one - but they are both helpful for benchmarking and measuring performance.
Six Sigma and the CMMI are completely different (although often confused) animals. Companies seeking Sig Sigma focus on defects and corrective action, with the hope of being able to correct the cause of the defect and eliminate it in future releases. The focus is largely on metrics.
CMMI is a process model intended to guide organizations through a cycle that improves productivity and performance, eliminates waste, reduces defects, and gives management much better visibility into project performance. The CMMI's focus is both process and metrics.
There is no real conflict between the two except when you realize that in order to predict defects and take true corrective action you must be performing and CMMI Levels 4/5. Many companies attempt Six Sigma programs long before they are mature enough, often leading to failure and wasted effort. Without the standard processes implied by the CMMI, it would be pretty tough to identify the process cause of a defect.
That said, neither has a “certification” per say. A company “achieves” CMMI Level 2/3/4/5 and “achieves” 3/4/5/6 Sigma.
In general, I think the CMMI Appraisal is a much stronger vehicle for evaluating the capabilities of a company, as it is a “best practices” model that used external appraisers to conduct the assessment.
CMMI is verifiable (by the SEI), whereas Six Sigma really isn’t. Any company could claim to have achieved SixSigma, with CMMI you are "awarded" a "rating" by a Lead Appraiser, and you can verify it on the SEI's web site.
As far as market position, I think it depends on the industry. In defense and manufacturing both SixSigma and CMMI are common, but no one is requiring companies to adopt SixSigma that I know of. The same is not true for CMMI. Many OEM's and defense agencies (as well as electonics, health care, and insurance) are requiring that their suppliers achieve CMMI Level Two as a minimum.
If you're in one of those categories, or you want to sell to one of them, the CMMI might be your best route.