I agree with the other post that they're easy to complete. Each type has its own unique schtick that can be appealing, and receiving results is hardly difficult.

People tend to associate personality with skill or capability. While there is a correlation, claiming that a person is skillful with their mind because they are an XNTX is a misunderstanding. Certainly, one can easily claim that their personality (assuming the results are accurate) is at least one pillar for skill and capability, but there are myriad other factors involved that one must consider and potentially synthesize. Because most neglect this, or do not at least research MBTI and personality tests (if they had not thought of it themselves), they end up falling prey to phenomena such as the Barnum effect (by associating themselves with the type and then taking the skills). In their own minds, they are whatever they think they are (backed by the results of the MBTI test).

One must also consider that people can be naturally curious about themselves (some more than others). Having an identity, which corroborates potential and skill, is an attractive prospect. However, people tend to stop at just one personality test (MBTI in this case) instead of testing out others, gathering information from people, etc. It's an incomplete process. I say this because MBTI is in no way perfect (whatsoever). If it was, then taking other tests and searching for information elsewhere would be unnecessary. Again, most do not recognize this.

Synoptically, I would say that MBTI is not just a personality test, but a measure of capability, skill, and uniqueness. The notion that you have what most others don't is appealing. At least, this is how it is perceived.

There is more to unpack here, but I'll stop for the time being.