If the first leg of a multi-hop flight is delayed, then the passengers on it may miss their connection. Then airlines put them on the next flight to the same destination. Or if the next flight is full, then the one after next, etc.

On the other hand, airports recommend that passengers arrive hours before their flight departs, in order to leave time for getting through security and to the gate. If the security line is short, then some people arrive at the departure area before the previous flight to their destination leaves. They would benefit from switching from their original flight to the previous one, because the wait at the airport would be shorter and they would arrive at their destination sooner. Currently, switching to an earlier flight is not allowed. However, the airlines would benefit from putting passengers on an earlier flight (if not full), because this leaves more seats open on the later flight. These seats can then be allocated to the delayed passengers from connecting flights.

The obvious question is, what if the delayed passengers need seats on the earlier flight, not the one from which people were switched to the earlier flight? The switch to a preceding flight can be decided at the departure gate just before this preceding flight leaves. In this case, any delayed passengers would already have been moved to this flight. Any optional switches of early-arriving people would have lower priority and a later decision time, so would not take seats away from any original or delayed passengers.

For the first flight of the day, the delayed people from previous flights are known hours in advance. The passengers booked on the second flight of the day to a given destination may be put on the first flight there, if they arrive before it departs. There is no worry about last-minute arrivals who may need additional seats on the first flight.