The characteristics of stainless steel are given by its composition, mainly by significant chrome contents. It is protected by a film of chromium oxide developed by influence of atmospheric oxygen. That means that stainless steel does not need any further coat for protection against corrosion.
In the course of standard manufacturing processes like cutting, bending, etc., the protective film is broken. Then, welding causes colour changes and corrosion in the welding spot, mainly in wet environment with presence of chlorides. Staining gets the stainless steel surface rid of oxidation products, as well as marking from suppliers and other types of pollution. The resulting surface is unified, with silver grey colour.