why they have to call hyung? is it that necessary

In Korea its out of respect, males call other males that are older then them that. But, when they dont use hyung that means that they are addressing an older person casually - which you don't use hyung unless your REALLY REALLY close. Even irl mlm relationship in Korea they still address the older male as hyung

They use it too when two men is in some kind of relationship like an endearment i guess.