They make the transportation more difficult by having lots of rain, having deserts in their rain shadow, provide urban water supplies and are the homes to indigenous people.
Like these are difficult to cross, separating the coastal plains near the Pacific Ocean from the eastern plains that extend into the Atlantic Ocean make communication between populations difficult.
Explanation:
As it is difficult to cross them, it is difficult to trade goods or supply them in a war. This type of population ends up being isolated from the world around them.
