MBTI comes in handy when you can read other people’s trivial character traits, interests and motives, so you can predict their actions and approach them in a specific way, depending on if you want to flatter them or make them ragequit a conversation in humility. The problem is that most people are lazy or not smart enough to educate themselves on how this typology works.
Taking a test won’t give you an accurate result of your type, unless you know yourself very well and can answer all of the questions truthfully and according to your senses - many underdeveloped people with huge ego lie on their test in fear of discrediting themselves or answer in a way how they want themselves to be, rather than how they’d instinctively act in a certain situation.
Typing someone based solely on “letters” won’t determine their type as well - it doesn’t always mean that a person is Introverted if he doesn’t like to socialize, as well as it doesn’t always mean a person is Extraverted if he won’t shut the fuck up.
Each type also has its dominant and inferior functions, like INFP-types being predominantly driven by introverted feelings, contrary to INFJ-types, which are driven by introverted intuition. They might share some common traits, like being disassociated with reality or experiencing an urge to please somebody, but you’ll notice how different they are as soon as you’ll ask them about their views or hobbies - if a J-type would be into the collection of extremely rare or expensive stuff, like exotic knives or vintage musical instruments, the only thing a P-type would be able to show off is his collection of keyboard caps or rocks shaped like food.