Ever since I researched into Myers-Briggs, I've considered it to be the equivalent of a Buzzfeed quiz ("Which Star Wars character are you?"). Entertaining, perhaps even insightful on a superficial level, but not at all useful.
It’s not useful to me because it’s not accurate. I already knew everything it told me about myself and recognized the parts that weren’t accurate, but what can I do with that information? I can’t expect that its conclusions about anyone else or how to relate to them are any more accurate.
I use them as Bayesian priors in my model of a person's behavior.
In my experience, when someone tells you they are x personality type, that's generally a starting point for you to refine your understanding of them as opposed to starting from zero. Unless they had meant to throw me off altogether, I generally find that information helps my mental model of them to converge fairly quickly.
Example: you ask a girl out and she tells you she self-identifies as INTJ, so you think to yourself -- she gets her energy by being alone or with a few people, she's probably a little cerebral and probably tends toward orderliness. You then have a basis of testing your hypothesis to see what her actual preferences are (vs stated), and also you might not want to plan your dates around meeting lots of strangers and doing crazy stuff. Remember, the utility of the MBTI isn't so much whether it is actually accurate or not -- it is more what it communicates. When someone gives you their type, it's them communicating to you a compressed signal that says, "I think I kinda fit this description" -- and that's a useful signal.
You may later learn that your model of her is wrong and needs refinement, but you'd be making iterations in small steps as opposed to throwing the entire model out. Once you know her, you no longer need MBTI or whatever, but during the initial stages, it definitely helps guide the exploration.
How is that not useful?
Sure, there are more complex instruments like Big5 and such that are more "accurate" but outside of research circles, no one uses them. That right there is an example of a more accurate, but less (day-to-day) useful signal.
(though I would say Big5 is good for understanding where you are on certain dimensions with respect to the general population. And it is undeniably useful in psychology.)
Prior to the widespread adoption of the internet (thinking late 90s) this idea that people's personalities could easily be grouped into distinctive categories was not a widespread idea yet. Buzzfeed "which star wars character are you" quizzes probably grew out of this, but that's tangential. Knowing the category exists is not nearly as beneficial as understanding what makes that category separate and how to interact with those types of people.
Most of the value of loosely grouping people together into groups is understanding how to interact with those people, namely how to approach them about an idea, or how to handle their reaction to negative news, or, why they reacted to something differently than you expected.
In school we studied "True Colors" which breaks personality types down into four primary types, or colors, with strong or weak secondary types. Most people on this forum would be classified as "Green". Then it breaks down how and why those four groups respond to things. This has been hugely helpful in my professional as well as personal life.
Another example of this sort of personality categorization is "The five love languages" by Gary Chapman which breaks down how people share and receive intimacy, which breaks down common things like why some women don't care about getting flowers but for others it's crucially important, and more importantly, why.
Many such online quizzes (also: what is your X name, based on your first & last names, birth year, city of residence, etc.) are very thinly disguised deanonymisation / classification systems.
There are 33 bits required to uniquely identify any individual on Earth. Since the userbase of any online service is already a far smaller subset (particularly when looking at active userbases), answering such a quiz (truthfully) is quite likely a way to specifically identify you. Even as little information as residential and work postal codes is effective at uniquely identifying 90% of a population.
The slightly less nefarious use is in classifying audiences for advertisers, though even this can be abused.
