The audio transmission process is quite different between traditional analog radio and modern digital radio. Analog radio sends signals in the form of sound waves in order to deliver audio, while digital radio transmits and receives by processing sounds into packets of numbers – increasing audio deliverability and quality.
With analog, you will get a noise floor (ambient static) in the background. The noise-to-signal ratio increases the further you get away from the transmitting radio, until eventually the audio coverage is unusable due to overpowering static. With digital, there is no noise floor. The audio stays clear and constant until the radio is out of range, yielding a significant increase in usable audio coverage.