Answer:
Many people in the book are superstitious and believe it's bad luck to leave addresses or messages when going into battle. One example is Eppie who feels that "it is unlucky to write a letter leaving behind a way of finding me". The author uses evidence from the text to support this answer, such as when Otiluke says that "all these symbols on the floor" represent battles and following them was "the secret of true success in arms. Do not ever walk across the floor...you will know why when you come to read the book of horrors."
Explanation: Another example is when Otiluke says "if you must write an address, write it in reverse. The wind will not blow it away if it is written that way."
Since leaving behind addresses or messages when going into battle makes one vulnerable the author uses evidence from the text to support this answer.