Reasons of tarnishing are the exposure to air, the corrosion and the metallic abrasion.
It is a common method to tarnish the silver, on purpose, to create another silverjewelry style. Liver of sulfur (potassium sulfide) is applied on the silver and as its color is black, this practice gives the jewelry an antique look. The old is always valuable, so is the old looking jewelry.
By Oxidizing, the indentations or grooves (recesses) are filled with black patina and the details are exposed. Besides, Oxidizing of a piece of silverjewelry, proves its purity and reality.
It is so easy to clean the Oxidizedjewelry, just by mixing dishwashing liquid and warm water and leaving the jewelry in that mixture for a short time, without concern for darkening.