Officially, talent wears two mics in case one mic or wireless should fail during the broadcast. However, that rarely happens, providing that the mic and radio have been tested before going on air, and a fresh battery inserted in the transmitter.
Most of the time, the only reason that talent wears double mics is because it is a status symbol, and implies that their performance is as important as that of the POTUS (who started that trend a long time back).