Throwaway ticketing is a simple concept wrapped in complicated airline logic. It happens when a traveler books a round-trip flight, but only uses the outbound leg throwing away the return. Here's an example: You want to fly from New York to London one way. The one-way ticket costs $850. But a round-trip from New York to London and back costs $620. So what do you do? You book the round-trip and skip the return flight.
I think it's due to the economic issues where money is uncertain. People still want to travel, and some have to for reasons they can't avoid, so they're inclined to find discounts.