It seems to me that to have a little clarity here, first we need to understand that what we're really talking about in this thread is Jungian typology theory in general. Socionics is one particular branch within this field. So, then, the question is, what is Jungian typology theory good for? The answer is, it helps explain phenomena about people's experience of reality that people are aware of but find difficult to articulate. In that sense, it is very useful.

However if you're trying to draw some easy, quick benefit from it, that's where you can get into trouble. Clearly, typology theory isn't a great way to find out what you're good at. Why? Because there are many more direct ways. To find out if you're good at math, a better way is to take a math class and see how well you do at it. If you ace all the tests without even trying that hard, then you're good at math, and it doesn't matter what type you are.

But typology theory still has many potential benefits, and they're limited only by one's imagination. Here's one: If you're correctly typed, and you're able to read about successful people of a similar type, you may be able to learn from how those people coped with similar problems that you face. Anyhow, that's just one of many possible applications.

One valid criticism of Socionics specifically is that it doesn't appear to place as much emphasis on how to apply it as other schools of typology, such as MBTI. Generally, MBTI-trained people will talk about the importance of understanding the strengths of those people who one previously saw only in terms of their weaknesses. They also put a lot of emphasis on using the theory to help "understand where people are coming from." I'm not saying that MBTI always delivers on it's promise, but there is perhaps more emphasis on applications and on how to talk about type in a constructive manner.

With Socionics, probably the biggest stated application is finding those types that one is supposed to have a particularly profitable relationship with...e.g., dual, etc.....although that goal is elusive to many people because the likelihood of mistyping oneself is so high when using the Socionics definitions.