Explanation:

Kalinga was a state (presently in Orissa) situated between the rivers Mahanadi and Godawari, on the coast of Bay of Bengal. This was an independent and powerful kingdom during the time of Ashoka.
Its rich and fertile land could contribute to the economic prosperity of the empire was not one of the amongst reason for its conquest.
The conquest of Kalinga was of importance to Mauryan empire both from strategic and economic point of view. It stood in the way of the southern point of view. It stood in the way of the southern routes from the Ganges valley, and it was also a powerful maritime area, which if included could become a convenient source of income to the Mauryan empire. After the conquest of Kalinga further territorial conquest was almost unnecessary for Ashoka.