The history of board certification is really interesting. Like many things, it's more evolved than designed.
In the early 1900's there was little standardization in physician training. Most graduating medical students did a 1 year rotating internship, and then became "General Practitioners". As such, they could do absolutely anything they want - surgery (of any type), procedures, etc. And most states didn't require an internship, so nothing stopped someone from graduating from medical school and just going out and doing anything they wanted. Much of what we consider modern medicine wasn't widely available - labs, xrays, etc. The world was very different.
Early 1900's heralded several change events. One was Flexner in 1910 which reformed undergrad medical education -- standardized it, and moved it from for-profit schools to universities. There was a similar push to standardize GME, although not as organized. The next big event was WW I. The army decided to classify physicians into various specialty sections -- based on whatever criteria they decided. Once the war was over, medicine decided that they wanted a way to self define "specialists" from GP's. And the boards were born.
But there was no overarching organization, and various boards focused on different goals. The ABS (surgery) wanted to standardize surgical training and experience, and was worried that patients "
...with intelligent judgement in other matters were cheerfully hopping up on operating tables and allowing a medical school graduate with one year of training in a rotating internship to peer and search aimlessly within their abdominal and other body cavities" Which was exactly what was happening. So they designed board certification as a floor - proof that you have the minimum skill set to practice as a surgeon. Meanwhile, the ABIM wanted board cert to be "a feather in your cap" - and designed board certification to be an aspirational goal, demonstrating mastery of the subject. They had no interest in most GP's pursuing board cert.
There was lots of infighting as to whether this Board Cert thing (especially in IM) was of any value. Many felt it was hooey.
And then along came another change event. WW II. This impacted physicians in several ways. First, WWII was the first big mobilization of physicians into the armed forces. And second, all of these GP's who were drafted into the army all of a sudden got exposed to the tools of modern medicine - xrays, labs, IV meds, etc. before this, much of that was out of reach of most GP's. The war (and the army) brought all that technology right to the battle lines, and physicians got all sorts of experience they didn't have before.
The army had one other big impact, likely completed unplanned. The army needs grunts and leaders. It's very hierarchical. So with all these doctors, they needed some way to decide whom was in charge, and whom was a grunt. And they had no way to do that. So they looked around, and found this "board certification" thing. And they decided that the docs that were BC would be in charge / higher rank, and those that were not would be grunts. And so, all these young docs coming out of medical school and directly into military service quickly learned that the way to get ahead was to be BC.
Post WWII, many young docs returned to residency spots. The gov't was now funding them, and the VA system was being built. All of the tools used by medicine in WWII became much more available. The GI Bill paid for much of it. ABIM shifted to a minimum competency model as more and more docs wanted board certification. Residency training became more standardized. And we were off to the races.
Should you be interested in more:
The invention and development of American internal medicine - PubMed
