The proverb "those who can, do; those who can't, teach" is usually seen as disparage for teachers. If you are not skilled enough at something to make a career out of it to yourself your only option is to teach it. But there is another way to look at it.
Teaching others is the best way and motivation to learn something new. To be able to teach something you really need to know your topic really well because you need to be able to explain it to somebody who knows even less than you do. If you want to learn something really well you should try teaching it to somebody else.
Doing what you are good at is an easy way for a career. You only need to be good enough to understand what you are doing yourself. You might even be able to discuss about your trade with others at the similar level. But the real test os to explain what you do to those with less experience.
Those who can, think they are good enough to do; those who teach know they don't know everything.