A possible reason for asking “If you had one superpower what would it be?” Would most likely be to figure out what is going on in that persons head. For example choosing to hear what other people are saying might play into wanting to hear what people have to say about them which might amount to an insecurity. Now invisibility might amount to something more serious, and it allows other people to understand how they are feeling. So asking this question might be for learning if that person is okay or just likes to dwell on superpowers and the good side of it. :)