There are many individual characteristics: physical (height, weight, etc), psychological, social, intellectual, etc. If we know where we are in the distribution of each characteristic across all humans, we can set realistic goals and have less problems. Sometimes neighbors can help us to locate ourselves in this space (but be careful, their opinion may be not objective, but driven by envy or ignorance), in other cases scientific tests can do it.
For example, a person knows that some exam puts him in top 10% in some science, and then pursues it. Or a girl winning model contest with top 3% result pursues a career of a model. In those cases there are tests that reveal objective truth. It is much more difficult to find out some psychological habits and traits of character that either are making you more successful in communications or ruin one’s other advantages.
If one has a wrong perception, he or she can invest too much in unrealistic goal. For example, trying to win some competition while having a very moderate talent in this area. Or to ignore the necessity to change some habit that destroys interaction with people. The result can be misfit in marriage market or problems with finding a job.
Any ideas both about personal experience in self-understanding or (non)-importance of this knowledge are welcome.