So FM ( Frequency Modulated) Radio signals are decaying signals in simple terms.
In technical terms it means that, every station broadcasts the signal with a power of 5Kw to 8Kw, depending on what city and location they are in and what kind of license they hold. This power of the signal decays as it travels the distance. Usually the signal from an FM station can travel upto 50–60Km sometimes even a maximum of 100 Km.
Beyond this distance the signal does not have enough strength to be useful and so if you go beyond that distance you will not hear anything on the Radio channel.
So, if you have to operate in several cities and you do not want to loose the brand value you have created using the frequency number, you bid and buy the license of using the same frequency in another city where FM stations are permitted.
This is how the FM stations that are run by major businesses houses are able to control the same frequency in different states.