I've done a few of these tests without the psychology background to fully understand them (i.e. Jung's methodology and the general science of psychology), so approaching the Jungian types from that position here's a partial argument:
I'm an INTP (hey look at that! I could have just said "I'm a Scorpio!") and it quite conveniently matches my software engineering profession - I'm supposed to be creative, inventive, and strive towards some ideal. Not that I wouldn't be satisfied with INFP, INFP, INTJ, ISFP, ISTP, ESTP, ISFJ, ISTJ.
It helps a lot that these letters came up as a score derived from my own answers, as it's emotionally difficult to argue with a complex crafted quiz when you are driving the answer. The individual types are rather meaningless: "introverted, intuitive, thinking, perceiving" and their opposites are just adjectives more or less relating to the answers you gave.
The meaning derived from this adjective soup is what's hard to swallow. How can there be actual predictive value in conclusions made from these types? Please, let's have some case studies, analysis, statistics, and facts which support these conclusions!
The "famous people of your type" only further adds to the confidence game. Out of a list of famous people surely you will be able to find someone you self-identify with! The fact that the "sensing" half of the type chart's population distribution is in the majority (I'm popular/normal), while the "intuitive" half is a minority (I'm special/superior) might also play on the test taker's confidence.
Personality types, like astrological signs, let you classify individuals you interact with and color your perceptions of them. It actually allows you to minimize or trash actual observations and any care an attention you have for the actual person and replace it with a wrote response tailored to their personality type.
People actually have personality consultations "career coaching" or "life coaching" quite readily employ personality type "analysis" - could that possibly be any more useful than a tarot card reading? Someone queue up a Pen & Teller episode please.
xpost to a few specialized communities