If the pilot finishes 20 minutes sooner as a stable number you can send out a new pilot and keep going or reduce airspeed and save fuel. If nothing else you can have a large cushion so delays don't propagate though the day. AKA your 10 min late at 10 am and your still 10 min late at 10pm. I suspect most large airplanes fly fairly complex routes so you can consider 30 min a day as 1 / 50th of a virtual airplane if you change which route each plane takes.