Start up and shutdown are when most of those niggly tech faults turn up. Keeping them running (like London black cabs until it was decided that running diesel engines for 18 hours a day, most of it idling, wasn’t that great for the air quality) cuts those down, most HDFM systems can do a download with rotors running for checking between flights. Also avoids the need for a turnround servicing between flights. In Abz, the main driver though is that many oil companies do not want their passengers on board during start up and/or shutdown.