Being able to track "where is this plane coming from" allows us to see whats happening up the line. Doing so, I have seen the airplane number change which has the effecting changing the reason for delay. For two hours waiting for a 737 from Hawaii, delayed due to mx. To a different plane from a different city delayed due to airport operations.

At a place like IAH they seem to be able to switch out the actual plane you are flying pretty easily.

In the above example the delay was >4h so they gave us $250.