Why would someone want to use verbal or physical threats to control another person?

Dominance or Fear and sometimes they maybe scared of the person they threaten

Explanation: Some people want to establish their dominance because of a secrect they may know, to take control of someone

Fear to take control of someone who is afraid of what you may be saying most likely something they don't want to be exposed  

Some people are afraid of their secrets being exposed so they may invoke physical or verbal threat to keep their secrets safe

b) to encourage free trade

i am confused, what is the question? i can you out if you put it into more depth.