Why there is caste & caste system

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I wonder why there is caste & caste system........in INDIA I think it is more .......I DONT have any idea about rest part of the world.. is it an old tradition .....people now very much educated dont belive in caste system....we see there are inter caste marriages happen,........but there are also people who are very strong on the caste system..........do people mostly use caste system for the benifit of people or for personel benifits