why is it advantageous to roast a sulphide ore to the oxide before reduction

It is easier to obtain metals from their oxides rather than sulphides. So the ore is first converted into an oxide. A sulphide ore is converted into oxide ore by roasting.

 Roasting process 
It is the process of heating the concentrated ore in the presence of excess air. Volatile impurities also get expelled. 
Zinc sulphide in its ore zinc blende ( ZnS) is roasted to obtain zinc oxide (ZnO). Mercuric oxide is obtained by roasting cinnabar (HgS) in air.