Is six sigma necessary for any organization, if yes then why?

The Six Sigma process  is an improvement system used to develop new processes or products at Six Sigma quality levels. It can also be employed if a current process requires more than just incremental improvement.

This is applicable where large production companies focus on both cost effectiveness and quality  and it is not necessary for small businesses

I beleive what is necessary is 'Competitive Advantage.

what any business is looking for is 'Profit" and "More Money'. even NPO needs the profit to sustain and survive.

so, what is really necessary is the "Competitive Startegy".

Six Sigma is a tool to lower the cost of process and the cost of defects "which mean better net income" and to increase quality of your product "which mean more satisfied customers which mean retain customer and sustained profit"

do you need6 sigma,3 sigma or even zero defect, it depends on your market.