In the days of deregulation - all tickets were paper and they had cash value. There were two tickets prices - First Class and Coach. All tickets were refundable. The holder of the ticket had the cash value.
You could walk from the airline you were booked on and go to a competing airline and hand them your paper ticket and get a new boarding pass on that airline. So if your Eastern flight was going to be delayed - you just said OK - and walk over to DL and hand them your paper ticket and off you went.
The differences between the airlines were service and food offerings. The airlines had 5 star dining and other gimmicks.