Answer: To allow trade and transportation. To provide easy access to food and drink. To grow crops in rich, fertile soil.
Explanation: Rivers have fresh drinking water and the areas near them are fertile and fit for agriculture.
The main reason why the Han dynasty and the Western Roman Empire collapsed was immigration. The Hun dynasty suffered emigration of nomadic people to northern China and the Western Roman Empire was invaded by Germanic invaders (Vandals, Ostrogoths, and Visigoths).
The black land consisted of fertile farming land created by the inundation of the Nile River and the depositing of silt. The red land consisted of deserts that surrounded the country and provided protection from enemies.