For example, how Mallory can gain power over Alice and Bob who don't directly communicate.
First, Mallory tells Alice: "I am stronger than you. If you don't do what I say I'll kill you." So Alice decides to submit.
Then, Mallory takes Alice to Bob and says to him: "Together with Alice we are stronger than you, and kill you if you don't do what I want." So Bob submits.
This only works if Bob doesn't talk to Alice, otherwise they would together see the empty threat.
With more people, it becomes a bit trickier but hopefully you see how blocking communication helps to gain power over people. Often the blocking of communication is done through propaganda, e.g. Mallory might entice Alice and Bob to hate each other, so they wouldn't get together and figure he's the manipulator. Or Mallory might just kill someone randomly to motivate people not to talk to each other.
Anyway, this shows the importance of free speech (which is really a misnomer; it should be called a right to listen). If you can freely talk (and listen) to other people, you reduce your chances to being subjugated by someone.
(This is also why in a war, people are incentivized to dehumanize the other party and not to listen what they say, because together they could figure out that the real enemy are the war profiteers on both sides, colluding. And besides, killing is the most reliable way of silencing someone. That's why people should also universally reject killing - based on someone else's word - unless they are directly threatened themselves.)