The Myer-Briggs Type Indicator (MBTI) was developed in the 40s as a test to help women, entering the work force for the first time, to find work/jobs best suited to their preferences.
"The research that Myers and her colleagues conducted in those early years was primarily interested in what kind of profession was best suited to each type. Indeed, one of the main goals in developing the MBTI was to help 1940s women, many of whom had never had a career, find satisfactory employment during the Second World War."
It was later used by career counselors to help guide college students in selecting majors that they would find interesting and that they felt that would excel at due to their intrinsic motivations.
After creating the test, it was given to college students and the results compared statistically to determine where the heavier concentration of types were found in terms of college majors. That is the basis for career counselor's recommendations for potential career interest areas based upon the type that the MBTI had indicated for a given individual.
Where all the other assumed traits of the 16 types comes from is anybody's guess.
I tested as INFP but do not relate to being a Mediator, a poet, or an altruistic guy ready to help a good cause.
But I did find career satisfaction in the kinds of activities that the MBTI indicates I would. Drawing (in Engineering Depts., creativity in designing and programming Computer drawing applications.)