Earlier quoted context omitted.
What about when you want to get to/from the airport or other transportation hub like a train station or bus station? One experience I had was I called the taxi company the day prior to my trip to reserve a taxi to the airport, but it did not show up, and I had to scramble.
What kind of airport doesn't have a convenient transit connection already? Actually, don't answer that question because I live close-ish to IAD and... it's a whole thing. But Dulles is located in a car-dependent asphalt wasteland so I think my point still holds. DCA is actually sane about this and is right off a major subway line (Blue).
One example is SFO from South Bay. Add in people with families, and public transit is out of the question.