Why should the metal sulphides and carbonates be converted to metal oxides in the process of extraction of metal from them? 

Explanation

This is because it is easier to obtain metals from their oxides (by reduction) than from carbonate or sulphide ores. So, before reduction the ore must be converted into metal oxide which can then be reduced. 
Caicination or roasting is done depending on the nature of ore, to convert the ore into oxide. A-carbonate ore is converted into oxide by caicination whereas a sulphide ore is converteq into oxide by roasting.