. . . why do six sigma? . . .

There are many reasons why companies use and adopt Six Sigma. Here just a few mentioned over and over from customers:

Reduced Defects and Costs. By using fine-tuned methods* and tools** companies are capable of identifying the sources of defects and dramatically reducing them. By reducing the cost of poor quality (scrap, rework, etc) companies improve quality and reduce costs
Improved Customer Satisfaction. Companies can create very satisfied customers by listening systematically to customers (Voice of the customer), translating their requirements into measurable data (Critical to Quality) and then using this data to improving products, services and processes
Higher Employee Satisfaction. By gathering improvement ideas of employees and empowering them to change processes in order to perform better, employee satisfaction rate increases dramatically. Employees move from “victims of a system” to “creators of value”.
Competitive Advantage. Due to better quality (defined by the customer), increased satisfaction (perceived by the customer) and new products and services, companies are creating a major competitive advantage.

Higher Growth. By investing increased profits (from decreased costs and additional sales ) on new products, services and processes using methods like Design for Six Sigma, the probability of growing faster increases exponentially.
Real Working and Flexible System. By systematically combining the understanding of customer needs with statistical analysis, revitalized human resources and data based management, leaders manage the transformation of their organization’s culture and create best-in-class companies.

*Business Process Management System, with Balanced Score Card / Business Improvement with (DMAIC) Define, Measure, Analyze, Improve and Control Method / Business (Re-) Design with (DMADV) Define, Measure, Analyze, Design and Verify Method / Lean Systems Methodologies

**E.g. Cause-and-Effect, Pareto, Affinity, Tree-Diagram, Driving Forces Diagram, etc.