Years ago it was common to be able to request a credit when you bought a ticket, and the fare for the same flights dropped. Airlines used this to give you confidence buying and not waiting. You could lock in a price, protecting yourself against price increases, while not worrying that you might be overpaying.
Of course most people didn’t claim any money back. Most people buy a ticket and don’t check the price again later. Services popped up to automate this, and airlines because less customer-friendly. However several airlines are still fairly flexible giving you credits back.