Whatever rare knowledge you have, it is something valuable even if it is just a niche. The more you study on certain area, the more you would become wiser in that particular area, just like surgeons. You can make money and have a better job and life with your knowledge and that is why people say knowledge is power.
Rare knowledge like psychic, astrology and numerology is very fascinating for some people while can be annoyable for scientist or astronomers. In a country where everyone believes that your name or date of birth could bring you luck in terms of wealth, health, love and marriage, you should learn numerology or astrology, this knowledge is power.
In a country where military forces is in power, than knowledge in war strategies is an advantage for you to get the best job in the upper level. The strength of your physical body is a bonus with the knowledge, but body strength alone can only help you to get a job as private and take you longer time to get promoted.
If you learn other languages than your mother tongue language is also a power. In my country if you could speak chinese, you could get jobs easily, you could buy things cheaper and you could get promotion in a very short time if your job performance is good or above average. So here chinese language is the power.