Even though neither had any background in psychology, Myers and Briggs enjoyed great success with their system. As Mrs. Briggs was getting quite old, Isabel Myers was the main driving force. Her initial idea was that certain personality types would more easily excel at different jobs, and the tool was intended to be used by women entering the workforce during World War II. However, it was not published until 1962, but since that time, it's become the most widely used basic psychology test. It's most often used outside of the psychological profession, and is employed in career counseling, sports coaching, marriage counseling, dating, professional development, and almost every other field where people hope to be fit with a role that would work best for them.
So the MBTI's practical use is overwhelmingly unscientific, and it's often criticized for this. Criticism ranges from the pragmatic fact that neither Jung nor Myers and Briggs ever employed scientific studies to develop or test these concepts, relying instead on their own observations, anecdotes, and intuitions; all the way to charges that your MBTI score is hardly more meaningful than your zodiac sign.