The meaning of that is that often written explanations and requests can be more persuasive than oral arguments or actual battles or fights.
The use of the written word is very powerful. Those reading what is written can learn, think about what they read, and use it to change the world or society. The sword destroys rather than changing the world through persuasion, and any triumph with the sword just ends in resentment, not willing acquiescence. That makes the written word more powerful. Good examples are the Bible, the Constitution, as well as various books and letters.