Many years ago I was an a Delta Air Lines flight from Tokyo to Atlanta. Of the crew that had rotated in two days ago, one of the pilots had become ill (stuff happens) and was unable to fly on the day the pilot was scheduled to fly our flight to the US.
As the flight is scheduled as more hours than any on-duty pilot can work, an extra pilot was necessary to be able to fly the route non-stop as scheduled.
So a technical stop was added (in Denver) to replace the crew.