Imagine you are off on holiday. Perhaps hiring a car to drive to your destination after catching a flight. You could put your ultimate destination in to Waze and the airport / flight you are going to take.

Waze could keep you up to date on your departure while you're driving to the airport. (and advise on a departure time from your origin). It can update your friends of your progress.

It could keep your ETA up to date with your friends by cross-referencing the flight information and expected arrival time. (It could probably show your approx location based on flight tracking data).

This approach might work for other multi-leg journeys like trains or ferries.