The MBTI is based on Carl Jung's work with psychological types. However, Jung's work led to the formation of analytic psychology. This work is often associated with clinical observations and anecdotes instead of controlled scientific study. This means that Jung didn't carry out research that can be considered conclusive and scientifically validated. However, I'm not sure if any of his theories can be or have been supported by research done by other people.
That said, the purpose of the MBTI is to be a scientific theory. Going back to its history, it was meant to determine jobs that would be most suitable for women entering the workforce in the United States during World War II. It's assumed that the person taking the assessment is answering the questions honestly. Also, the MBTI results don't indicate ability or aptitude, but preferences. The MBTI has been used by human resource departments and managers to gain some high level insights into their staff and potential methods for structuring organizations and leading teams. Employees have also used the MBTI for determining possible career directions that they might be more likely to have a preference for.