Korean people use 우거지 for many variations in cooking. 우거지국, is example, soaked up the waters seasoned with. The famous 감자탕 also has 우거지 in the ingredient.
우거지 is outer leaves or stems of napa cabbage, radish and other greens which are removed for trimming to get the core of vegetables. The outer leafs are dried in many days to preserve longer duration of times. It is known that the dried process increase nutritions. When it is put to soup boiling, it extends again and easy to immerse seasonings in the soup. It tastes good.
우거지 해장국 is friends of people who work early bird at the morning. Many working people go to restaurants before going to workplace. Or, sometimes people drank much alcohol, next day they eat 해장국, hang over purposes. 우거지 해장국 is one of the famous hang over soup taking.
The video at the above is Korean introductory culinary tour program introducing various restaurants in Korea. It would be informative for international people too.