when SS is not given purging, then root side of the weld gets oxidised due to formation of Chromium Oxide.
So we have to displaced the air inside SS pipe so the internal environment of pipe is free of Oxygen.

Same thing dosen't happen when welding MS, as there is fraction of Cr in composition.
But for Alloys having Cr greater than 2.25 %, Purging is given for the same reason.

Hope this could help you.