Stainless Steels need to be cleaned for aesthetic considerations and to preserve corrosion resistance. Stainless steel is protected from corrosion by a thin layer of chromium oxide.
Oxygen from the atmosphere combines with the chromium in the stainless steel to form this passive chromium oxide film that protects from further corrosion. Any contamination of the surface by dirt, or other material hinders this passivation process and traps corrosive agents, reducing corrosion protection. Thus some form of routine cleaning is necessary to preserve the appearance and integrity of the surface. Stainless steels are easily cleaned by many different methods.
They actually thrive with frequent cleaning and, unlike some other materials, it is impossible to 'wear out' stainless steel by excessive cleaning. Stainless Steel performs best when clean - cleanliness is essential for maximum resistance to corrosion.