I work in the service industry. My job requires that I have a specific skill set, writing code in a few different languages. From a job perspective, my ability to write code in those languages is of extreme importance.
However, from a career perspective, the ability to perform a core job function is not the valuable ability to have.
The greatest ability one can have is the ability to acquire new skills quickly.
This is especially true in the service industry, but it can be applied anywhere. You never know when you will have to step beyond your job description to save the day. Without a doubt, this is also a very important ability for business owners.
Doing your job well is one thing. Saving the day is another.
Exercise
This is from a programming perspective, but you can apply the concept to your own industry. As a programmer, I must stay up to date and at the top of my game when it comes to the platforms in my core skill set. However, researching, adopting, and practicing different technologies is also a must. If you try out new languages regularly, your mind will adapt to learning new syntaxes, new methodologies, and making sense of things that initially seem foreign.
The result is a mind that is versatile and (to your employer and/or clients) valuable.