I had few delays like OP said, it was first flight of the day at DFW, ORD, LAX, after passenger boarded a pilot told us that they found mechanical and mechanics need to look at it. I thought same as OP that why they could not take care of this overnight? It was not at out post, it was at major hub. How I knew aircraft did not just flew in on red eye flight? I tend to check incoming aircraft to see if my flight is on time or not, I do not trust "On Time" display on flight status. I knew aircraft flew in night before and parked at an airport.

I think my experience with "Pointless Delay" was at DFW. Arrived at the gate, pilots and FAs were at the gate also but there was no aircraft at the gate. Closer to departure time a pilot made an announcement at the gate and told us the situation. Our aircraft was at DFW, aircraft flew into DFW evening before and was parked at a maintenance area. But they forgot to assign a ground crew to tow aircraft to the gate, they could not find anybody to tow our plane to the gate. Ended up an hour plus delay.