When a wild animal senses a predator, it rarely speaks out to frighten an enemy - it is on some occasions to give a quick shock to the enemy and stop it in it's tracks (I know cats jump backwards when they hear a sudden sound) but normally it's to alert all other animals to the presence of a threat. Birds and monkeys are a great example of this, and as chocobos are birds, well, yeah! Still, Wark isn't only used in battle situations, it's used by wild chocobos (including those you race in FFVII) when they are happy etc. as well!