Olympians bite their medals to satisfy the photographers at the Olympics. Many photographers at Olympic events believe the shot of Olympians biting their medals to be an iconic shot that they must have.
Historically, biting Olympic medals and other precious medals stems from the act of counterfeiting. People who handled money many years ago bit down on the money to be sure that it was not counterfeit. Gold is a soft metal that shows bite marks and distress when bitten. The marks proved to money handlers that the money in question was genuine. As of 2014, money does not contain precious metals, so there is no longer any reason to perform the bite test.