Control charts permit us to determine the state of control of a process. A second important point would be the ability of the predictable process to produce products which meet specification requirements. A reduction in the variation of a properly centered process will make the process more capable of producing products which meet specifications. An alternative way to improve the capability of a process would be to expand the specification limits. Often you will be constrained from doing this by customer requirements.
Variation is the enemy of quality.
It is the variation that is inherent in a process that is operating as designed. Assignable cause variation is unnatural variation in a process. It should be identified and addressed. Control charts enable us to determine the type of variation that exists in a process. A common problem in business is managers who treat all variation as if it were due to assignable causes.