Alexander added that the flight would have left with or without him so he was not creating a disproportionately negative environmental impact by getting on the plane.
According to the Wall Street Journal, there are a variety of reasons they airlines decide to proceed with flights that are severely under-booked. They occur if passengers on board have essential business or are attending key life events like funerals.
Apart from the passengers (or passenger) on board, flight timetables rely on planes, crew and pilots, being in specific locations at specified times and their next journeys may have considerably more passengers.
Flights also carry cargo as well as passengers and planes may need to get to a maintenance base for overnight routine work.