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

Why should the metal sulphides and carbonates be converted to metal oxides in the process of extraction of metal from them ?
Answer:
It is easier to obtain metal from its oxide, as compared from its sulphides and carbonates. So prior to reduction, sulphide ores are converted into oxides by roasting and carbonate ores by calcination.