When your flights are cancelled or substantially delayed, and the airline can’t get you where you’re going, sometimes you’re willing to… fly somewhere else. If you’re flying cross-country, you might even take something hundreds of miles away from your destination, if the trip is important enough. Maybe you’d fly to Los Angeles instead of San Francisco and drive.
An interesting question is, what if anything does an airline owe you if you do actually fly – but don’t fly where you were trying to go?