It must produce more business than it costs. The major advantage of radio is it's everywhere. Our cars, bathrooms, bedrooms, kitchens and the workplace. Try to find a jogger without a radio and a headset. No other medium reaches so many people in so many different places.
Few people listen to more than two or three radio stations.
In small towns it may be just one. Every age bracket registers in the high 90's as weekly listeners. Keep in mind that your local stations numbers may vary depending on the size of your town. Most large cites have people commuting to work by car or public transportation and many carry radios with them to pass the time while stuck in traffic. For advertising this is a truly captive audience.