Is there a reason to not add salt when making a soup?

On several occasions I have had friends make soup without salt and then tell me that salt should be added only when the soup is served.

I've read Why is it important to add salt during cooking? But what is the reason, if any, for withholding salt during cooking?

I have only heard this advice in context of Chinese soup/broth, namely a Chinese-style chicken soup and a brothy oxtail soup. I've asked a friend about the reason, but didn't quite understand the response (something about damaging the proteins).