Silver is the most prevalant material prescribed in Ayurveda to clean the tongue. It has a cooling effect on the mind and nervous system and helps to 'control' the tongue. This was the choice of Ancient Chinese Royalty and European Nobles Disease organisms cannot live in the presence of even minute traces of Silver. Silver minimizes the growth of plaque in the mouth. There is also subtle electromagnetic event that happens when you connect your two hands with your tongue via the silver, but this is a bit esoteric for now.
Silver removes bacteria better than other materials.
It is good for preventing sore throats, influenza, etc. This is more esoteric, but a good reason, nonetheless. The tongue is a great place to discern the state of the Agni, the digestive fire. So when you scrape your tongue in the morning you can monitor your agni. If the tongue is coated, agni is too low relative to the diet.