Why do you think Derinkuyu ,the underground city in Turkey,was built?

The underground city seemed to be very cleverly made and whoever made it knew what they were doing. The ventilation on the underground city seemed to be very well planned. It also seems that they did many other things in there like take care of livestock and worship the god that they believed in. So, I am guessing it wasn't just like a place they went when tornado's happened. I get the feeling that the people lived down in there. If they had a enemy attack them the enemy could easily smother them but covering up the vents. Why do you think Derinkuyu ,the underground city in Turkey,was built?