The FCC also controls how much power a transmitter can use so as to prevent interference. For example, let's say you've got a station in Conway, Ark., transmitting on 93.1 FM. Another channel in Memphis is also transmitting on that channel. The FCC can limit how much power, or watts, one or both stations can use so as to not interfere with the audiences of the respective stations.
The same regulations were in place for CB radios in the 1970s -- there was a popular technology with the potential to create all sorts of interference in the somewhat narrow frequency spectrum shared by a lot of users.