For example in a maintenance situation, flight arrives w mtc issue that warrants a ship swap, any pre-submitted data goes awry...

...Or at a hub where other aircraft scheduled for a later flight may be available for a sub on an earlier flight w a delayed inbound, mtc., or other issues.

I guess I'm too used to ATL being able to swap equipment fairly quickly (sometimes) & know enough folks who swap ships all day for a living to know I can't always rely on other sources as the definitive answer.