there's also the case of booking 2 seats on the same flight, then cancelling/redepositing the 2nd last minute for whatever reason (hold for some procrastinating friend etc). Tough to do these days with many overbooked flights, but otherwise same effect for the airline, lost revenue and risk of getting your account zeroed out. I did this one time to hold a seat for a friend who was booking later and wanted to sit together. completely forgot about it until boarding, the software had not canceled either ticket, but I was able to cancel from the app. This was maybe 8 years ago, before the days of every flight being oversold, so things might work differently now.