Mountains are advantageous in several ways. One reason is that areas with a variety of elevation have a greater variety of plant life. Places with mountains and valleys have more diverse plant life than regions made of flatlands and gentle slopes. Plants are important to a developing civilization, especially at times when wild game is scarce. Plants can also be used medicinally.
Mountains are also important in mining. Most mines are found in or around mountains, offering a plethora or raw minerals, like iron and silver.
Militarily, Mountains offer defense. They can be a significant barrier in terms of holding off invaders or even unwanted migrants.