From The TV IV
Must-carry is a regulation enacted by the Federal Communications Commission in the 1980s. The rule was created so that local stations would have a place on the growing cable television services without the threat of competing cable television content. The rule states that if a locally licensed terrestrial television station requests a local cable provider carry them on their service, the cable provider must comply and carry the station.
In an extra incentive to protect the local stations, the regulation also states that cable providers cannot carry any out-of-market television stations. The only exceptions are licensed superstations, which have been granted nationwide cable carriage by the FCC. One loophole that exists in the regulation is that if there is no local affiliate of a national television network, the cable provider is permitted to carry an out-of-market affiliate until the local affiliation is taken. DirecTV and Dish Network use New York and Los Angeles stations for this purpose. However, with the growing number of low-power TV stations and digital subchannels, the lack of individual network affiliations are starting to dwindle in each market. Many low power TV stations rely on the must-carry regulation so that they may get city-wide carriage, thereby offsetting their poor signal.