Some of the best themes to write / think about when it comes to this book is education, poverty, social inequality, and violence, but on the less gloomy side there's also a lot of themes of perseverance, and hope. These should apply well to chapter 8, Good luck!
The answer should be decadence
The answer would be "to help you decipher symbols and scale."
Without the key, a lot of the symbols would be quite confusing and you wouldn't know if it was 5 miles or 500 miles. Everything else is implied throughout the map, but the key's purpose is definitely to help you decipher symbols and scale.
