Because:
1. If there are some problems in the previous leg, resulting in delays (such as weather, aircraft failure, route control, etc. )
2. There is another situation, that is, this flight used to fly like this, but it just happens that the plane is scheduled for inspection on this day, or for other special reasons, there will be capacity allocation.
In order to reduce this situation and avoid large-scale delays, airlines will temporarily deploy planes from other places to take over the next flight task.