> There is NO benefit for customers from this at all that I can see.
Okay, lets assume it costs (all in, with all your overheads, taxes, etc.) $x to fly a plane from A to B, and it will carry y passengers. So if the airline can sell y seats at $x/y each, it breaks even. If it doesn't believe that's possible, it won't run the flight, no-one gets to fly from A to B.
So how about instead they sell a small number of seats at a vastly inflated price (we'll call it "Business class" shall we), in return for benefits that don't cost nearly as much as the price difference. Suddenly the rest of the seats can be sold more cheaply and the flight goes ahead still.
We can continue that, to differentiate people who kinda fancy going from A to B (ie for a holiday), from those who have to (ie for work). If you can persuade those who have to, to pay more, they can drop the prices for those who do not have to fly, again making it more likely the whole thing will go ahead.
If you ever actually have to price things, you very rapidly discover that many things are only cost effective with differential pricing depending on willingness to pay.