Question

why is it advised to switch off tv/radio during thunderstorm

Answer 1

during lightning or thunderstorms high voltage electricity gets released and when this high voltage electricity comes into contact with any wires or antennae thats not properly grounded it gets passed on and reaches the electrical/electronic appliances that are up and running and damages them. as most of these appliances are not capable of managing such high voltages. 

if you keep the radio/tv switched off during such times then it means you are preventing this high voltage electricity from passing on to your tv/radio and are safeguarding it from damage.