Wellesley established the Fort William College at Calcutta because

Lord Richard Wellesley, the Governor-General of India from 1798 to 1805, established the Fort William College at Calcutta. The purpose of this institution was to educate European administrators about the Oriental culture, tradition, law, and administration to better coordinate in the governance of India.

So, the primary purpose of establishing the Fort William College was to train British civilians for administrative purposes in India.This was part of Wellesley's broader strategy to strengthen the administrative capabilities of the British in India.