If you search in Wikipedia about “Process Improvement methodology”, you could find over 1000 of different methodologies and techniques (a.k.a, tools). There could be a lot of tools as many as the number of scholars and researchers.
I tried to apply as many as tools to my real projects, and figured out that such tools were very effective to solve a specific problem in a small scale project, but a big problem in a large scale project.
For example, even if a tool is very useful to solve a quality issue of particular product, the solution could make a negative impact to another team. Even the tool could not solve the root-causes of the problem which came from a defective design of the product. Overall, the tool could not solve any problem in the organization at all. It makes sense because the tool was used for solving the specif problem in a team, and not for solving a large problem in the organization.
What would happen if each team in the organization uses a different tool to solve a problem (i.e., a large problem for an organization, but a small problem for each team)? Can the organization solve the large problem? Probably the answer could be “NO”. The organization could lose overall consistency and create a hodgepodge solution like a Frankenstein.
The purpose of using a tool is to find a solution and optimize the solution for a local area (i.e, Local Optimization), and neither find a solution nor optimize the solution for a large area (i.e., Global Optimization). For the global optimization we could need to take another approach.
There are many approaches, and one of the best ways could be to apply a problem-solving framework to the organization. Lean Six Sigma is the best framework for problem solving, which solves an organization-wide issue and optimizes the solution globally with integrating many tools. That is the purpose of applying a problem-solving framework and the value of it.
Sometime people misunderstand that using tools itself is Lean Six Sigma. Indeed Lean Six Sigma uses many tools, but strictly speaking, Lean Six Sigma is the way of thinking, or the order of thinking. The purpose and the value of Lean Six Sigma is to solve an overall problem with global optimization, and sometime it is called as systematic thinking or scientific way of thinking.