It is a characteristic of korean language in vocabulary. There are many words originated from chinese in korean and we use two different type words because of a strong point of chinese alphabet. As you know , each chinese letter has it's own meaning, so we can describe something briefly . For example, '일,이,삼,사,오,육,칠,팔,구,십(一二三四五六七八九十)' is more simple than '하나,둘,셋,넷,다섯,여섯,일곱,여덟,아홉,열' or "원,투,쓰리,포,파이브,식스,세븐,에잇,나인,텐' especially,when we calculate or memorize big number , the former is faster and more advantageous.