Probably filling seats on unpopular routes that they would have to fly anyway. Perhaps in a similar situation to what's described in [this video] , where an airline was flying empty planes to hold their slot at Heathrow.
This existed pre-COVID but is more of a thing now. See Half as Interesting - Why Heathrow Airport Had Empty Flights to Nowhere (3 years ago, pre-covid) [this video] RealLifeLore2 also covered the same topic as the original submission: [link]
Pre-COVID explanation of empty flights to Heathrow to keep slots.
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