You're right that it's not intrinsically essential.
To an airline, a flight that leaves with an empty seat is basically throwing money away; that empty seat doesn't mean you can take less fuel, fewer flight crew members, or wear out the airplane less. Therefore, the empty seat saves the airline no money.
Furthermore, people often miss their flights.
Overbooking is one way to deal with these two assumptions. Assume a certain number of people will miss the flight, and sell some seats twice. Then you don't fly with money-eating empty seats, and you can lower the ticket price a bit, because you don't have to charge customers the cost of those statistically-likely empty seats.
There are other solutions. Don't offer any refunds or reaccommodation to no-shows. The seat was empty, but someone paid for it, so who cares? But customers probably don't like that. The might even like it less than overbooking. I don't know, I don't run an airline. There are options, but overbooking exists for a reason.
Well, there is more fuel left in the tank, so there is less on the refill so yes it does save fuel, not much but it does save some. I prefer no refunds for no-show policy myself, that is how it works when I buy a ticket to see a play. Now refundable upto 48 hours before the flight might be even fairer.
The fuel increase is marginal. You still have to burn fuel to carry that fuel to the next airport. And you have to carry the airframe with capacity for that empty seat. The 100 kg for a passenger+luggage is much less than the fractional weight of the plane required to carry them!
For some numbers, a 777-200 carries 313 passengers, weighs 135k kg, carries up to 100k kg of fuel, and has a max takeoff weight of about 250k kg. 313 passengers and luggage, averaging 100kg, weigh just 31.3k kg. Your ticket therefore pays for your weight, plus 5x your weight in airplane structure, plus (up to) 3x your weight in fuel.
If you don't show up, they're still carrying 8x your weight anyways, so saving 200kg isn't that important.
(PS: apologies for the 'thousands of kg' units. I thought it was more readable than Mg or fully written out numbers.)
> I prefer no refunds for no-show policy myself, that is how it works when I buy a ticket to see a play.
What about if you are flying long haul and linking up to a domestic flight on another airline? "Oh, we can't help you because your previous flight was delayed due to weather/mechanical/congestion.you should have flown on one carrier even if the price was 2x"?
Airport congestion is a big issue in the US, as is weather related delays.
There are absolutely circumstances where flights on different carriers end up on different itineraries. Sure, TS is one possible solution in the event of delays but so are processes that make dealing with delays out of the ongoing carrier's control more passenger friendly.
>that empty seat doesn't mean you can take less fuel
I was under the impression that the fuel load for each flight is calculated based on a formula that averages the weight for each passenger and their expected luggage. Fewer passengers than expected = more fuel left over after the flight.
If the 100-passenger airplane always had only 99 passengers, there would probably be noticeably lower fuel cost. But a 99-person airplane would be even cheaper.
To an airline, a flight that leaves with an empty seat is basically throwing money away; that empty seat doesn't mean you can take less fuel, fewer flight crew members, or wear out the airplane less. Therefore, the empty seat saves the airline no money.
Furthermore, people often miss their flights.
Overbooking is one way to deal with these two assumptions. Assume a certain number of people will miss the flight, and sell some seats twice. Then you don't fly with money-eating empty seats, and you can lower the ticket price a bit, because you don't have to charge customers the cost of those statistically-likely empty seats.
There are other solutions. Don't offer any refunds or reaccommodation to no-shows. The seat was empty, but someone paid for it, so who cares? But customers probably don't like that. The might even like it less than overbooking. I don't know, I don't run an airline. There are options, but overbooking exists for a reason.