During COVID a lot of empty flights flew because otherwise the airline could lose the gate slots.

I once flew on a flight from ORD to ROC where I was the only passenger. It was very, very weird to be in a big empty cabin all by myself. The flight attendant just came and did the safety briefing sitting next to me. I asked her why they didn't just cancel the flight, and she said the plane had to be in ROC for the next morning anyway. This was in the 1990s though. I've never encountered anything like that since.