Like most musicians, I’ve developed a unique set of specific skills over the years. For example, I can comfortably sight transpose lead sheets down a minor or major third. (Don’t ask me to do a fourth, or up a third.)
I can do it because I met a situation where I needed to do something I couldn’t do. In this case, it was that most fake books had songs around a third too high for my wife to sing. Compelled by a desire to make music with her, I acquired a specific skill.
I think most specific skills are like that. You are confronted with something that you can’t do, but which you need to do for a specific gig or other reason. So you acquire that skill.
Eventually all of these skills add up to a specific musician, with more individuality than the one who finished music skill with a collection of more universal skills provided by a formal curriculum.
I’m still acquiring small, specific skills – especially since I seek out jobs that don’t simply use the skills I’ve already got. Continuing to grow and evolve makes you more hirable, but also keeps you engaged in your career through curiosity and interesting practice.