A local television station carrying a national network is usually a separate company from that network. The relationship is contractual, and it defines who sells which advertising minutes.
Affiliation is a negotiated agreement
A network supplies programming to a station in a given market, and the station agrees to carry it at scheduled times, clearing the network's schedule.
Networks own stations directly in some large markets, but across most of the country the local outlet is independently owned and affiliated by contract.
Because agreements come up for renewal, a market can see an affiliation switch, in which a network's programming moves to a different channel entirely.
Advertising inventory is split
The economic core of the arrangement is that a program's commercial breaks are divided, with some spots sold nationally by the network and others by the station.
Local availabilities are what fund local operations, including news, and their value depends on the audience the network programming delivers.
This is why a strong network schedule matters to a station's finances even though the station did not produce any of it.
Retransmission fees changed the flow of money
Cable and satellite providers pay stations for the right to carry their signals, and stations in turn pay a share of that to their network.
These fees became a major revenue source, reducing dependence on advertising and giving stations an interest in how their signal is distributed.
Disputes between station groups and distributors occasionally result in a channel going dark during negotiations, which is a contract standoff rather than a technical failure.
Local news is the differentiator
Network programming is identical across markets, so the only content a station controls is its own, and that is overwhelmingly news and local sports.
News operations are expensive, requiring crews, vehicles and studios, but they generate inventory the station sells entirely on its own account.
Station groups have responded to cost pressure by sharing resources across markets and centralizing production, which is visible in how similar newscasts have become.
Ownership rules shape the map
Federal rules limit how many stations one company may own and how much of the national audience it may reach, and those limits have been revised repeatedly.
Consolidation within the limits produced large station groups that negotiate with networks and distributors from a stronger position than a single outlet could.
The regulatory framework continues to change, so the balance between networks, station groups and distributors is periodically renegotiated rather than settled.