In Japanese (and perhaps other far-east) culture, it is used as a term of respect when addressing an indiviual, usually someone older than yourself. It is akin the the english 'sir' in showing the proper respect for an individuals wisdom or office. So, if I were in Japan (and if I were actually worthly of any respect), I would be addressed as Xunker San.. though my name would probably be pronounced as 'Junke' (Yun'ka-e)