Here's an example of the idea:
It's spring, and you're planning your next vacation far in advance. You want to go to the bahammas for winter... but you really don't care (right now) when precisely you go, so long as it's within a particular range of dates. It's so far in the future, that you can
adjust your schedule to match the precise date of the flight.
You give the airline a list of a dozen different flights you'd be willing to go on, and they charge you some percent less (compared to what a regular ticket would cost) than the cheapest of the flights on that list.
A month or two after buying the ticket, the airline calls you back, and says, "we've scheduled you for flight X from your list."
You benefit from the discount, the airline benefits from being able to put you on the flight which is cheapest for them. Furthermore, if a particular flight has a very small (and thus unprofitable) number of people booked for it, and all those passengers had adapatable flyer tickets, the airline can cancel that flight, saving the cost of fuel, etc., and move those passengers to their alternate flights without needing to pay them compensation (since it's built into the discount of the ticket).