Knowing how many letters are in this sentence does not give me much power over you. However, knowing the numbers on your credit card certainly could.
It isn't quantity of knowledge that matters, it is quality of knowledge. The quality of knowledge is what gives people power. Often people have to sift through a great quantity to obtain the quality, but it does happen.
'For also knowledge itself is power'-Francis Bacon
If you have information of the world of finance and the markets and you also have insider information then you can become rich. The more money you have the more influence you can exert. Money = Power
This is an example of how knowledge can bring tangible results in the physical world.
Knowledge is often missused. Those who understand how the human mind works can exercise control over people, for example by manipulating their subconscious mind.
If they can control the flow of information to people they can then choose what information to give to people which in turn shapes how those people see the world. In effect they create people's perceptions.
If those people gain new knowledge it can create a door in their perception.
For many people their perception of reality is a lie. If they become aware of a new possibility in their understanding of reality then this possibility forms a door in their perception.
If they seek further knowledge that knowledge acts as a key. They can walk through the door and see the world with fresh eyes.
Their old ideas of reality fall away and are replaced with a new perception (which may also be a lie or an incomplete picture). They are more enlightened or illuminated.
Life is a journey through the doors of perception.
Those who want to exercise control over others can seek to create false perceptions, hide doors from people, hide the keys and deny the existence of certain possibilities.