The purpose of a WPS is to provide the welder with direction and to ensure there is consistency in the mechanical properties from one weld to another, from one welder to another.
Depending on the base metal and the application, the range of welding parameters may or may not have an affect on the properties of the completed weld.
In the case of carbon steel, unless notch toughness is a concern, heat input is of little concern. In the case of Q&T steels, heat input is a concern, so the permissible range of the voltage, current, and travel speed must be closely controlled. In the case of heat treatable aluminum such as 6061-T1, the interpass temperature must be controlled to minimize the time at temperature to mitigate overaging. When welding austenitic stainless steel the same is true regarding controlling interpass temperature, but for a different reason.
If every welder was a metallurgist and if every welder took the time to learn what base metal they were welding, the need for a WPS may not be as critical. However, not every welder is a metallurgist and most welders have little interest in the properties of the metal they are working with. Thus someone else has to do the leg work to research what procedures will most likely produce acceptable results.
Until the WPS is tried and test, one has little real knowledge of what the mechanical properties of the completed weld are. Thus we qualify the WPS and record what was done and what the results of the tests were. Then and only then do we know for sure whether the WPS is viable or just someone's best guess.
For many years most welders believed preheating drove the moisture out of the base metal, a necessary precaution to mitigated to probability of delayed cold cracking. For many years welders believed the secret to welding high strength steel (armor plate, etc.) was to use filler metals with high nickel content (309, 310 austenitic stainless steel, Ni-Rod, etc.). We now know the secret is to closely control hydrogen and cooling rates. It wasn't the production welder that figured out the solutions to those problems, it was engineers' research and experimenting that yielded the solutions. That information is passed to the welder via the WPS. The weak link is that the WPS doesn't tell the welder "why" he is directed to weld in a certain way.
A welder with a broad breadth of experience can "learn" by doing. Of course he will stumble along the way, experiencing failures as well as success, after all, that's what learning is all about. How many times does a baby fall and get up only to fall again before he learns to walk? In welding, we can't afford to wait until a welder learns by doing. There are lives and property at stake. The WPS includes all the lessons already learned by other people.
The difference between the experienced welder and the engineer is the engineer reads books that summarize someone else's experience. The experienced welder, if he's lucky and observant, learns by doing. There is nothing wrong with that approach other than it is slow, expensive, and often costs losses in terms of lives, property, and profit.
The smart welder doesn't simply depend on his personal experience, instead he reads books written by others that have taken the time to summarize their experiences and what they have learned.
As for the CWIs, like myself, many of us have experience as welders and we've studied the subject of welding. The two, experience and education, are essential to being successful. I worked as a welder while earning my degrees, and I continued to weld with a degree because welding paid more than the pencil and paper. However, the education made my job as a welder easier and I was more successful than many of my coworkers. In some cases they may have made "prettier weld beads", but as you know, there is more to welding than laying a pretty bead.
You were not born with the knowledge and experiece needed to make you the expert welder you are. It was only after years of plying your skills and learning hard lessons along the way you acquired the experience, knowledge, and expertise you now have. The problem is, now both you and I are "old." When we pass on, that knowledge, experience, and expertise goes with us unless we pass what we know on to those entering our field.
Be a mentor!
Best regards - Al
