I believe that it is a mechanism to "
segment the market". It is part and parcel of other mechanisms aimed at the same goal (advance purchase, standby fares, weekend and holiday rates, round trip around a weekend). In general, you want to give price breaks to the passengers who are price sensitive or are shopping around and not give price breaks to passengers who are not price sensitive or have no choice.
The classic case is an airline that has a hub in B. They have no or few competitors in B.
The airline can charge high rates for a flight from A to B. They are the only carrier that can service the route. But when taking passengers from A to C, another carrier with a hub in B' (B prime) can compete. The airline must reduce its fare or lose business.
In this case "segmenting the market" means locking A to B passengers into the A to B fare while allowing A to C passengers to use the lower A to C fare.