It was an old practice that used to be performed by athletes after winning the medals, earlier the gold medals for pure gold not just gold plated. Gold is a very soft metal and it is even softer than enamel that is present in teeth.
The practice of biting the gold medals was allowed to see if the metal that is used for making them is really gold or it is just gold plated? If athletes bite on a gold metal, they can leave some teeth marks in it by biting it. The gold medals used to be very soft that their gold plating could be scrapped off.
But now this practice has been obsolete for decades as the Olympic medals are no longer made of pure gold. In today’s time, they are made up of silver sterling or simply just gold plated. Of course, even silver is softer than enamel but it is not precious like gold.
The importance of this gesture has just become a regular pose for athletes and many athletes admitted that they pose for such photos because cameramen and journalists asked them to bite the medal, even they don't the real story behind this.