Why did Korea create the character 㔔, a character not used anywhere else but in Korea?

ANSWER: Because Koreans apparently needed a character to represent the Korean sound /덩/.

 But why would 㔔 represent the sound /덩/ instead of the sound /강/, given that its component character 加 is pronounced /가/, not /더/?

ANSWER: Because they used the pronunciation of the pure-Korean "meaning" of 加 (가), not the pronunciation of 加, and the pure-Korean meaning of 加 in 더, which means "more." And then they just needed to add the /ng/ sound of the Hangeul letter "ㅇ" to the character 加 to represent the Korean sound /덩/.

 But instead of 加 (가), why didn't they just use a Chinese character that is pronounced /더/ as the component character?

ANSWER: Because there is no Chinese character that is pronounced /더/.