As already stated, to avoid howlers.
An example of it working, is Vardy v West Ham - Lino thought he was off, waited until the phase of play had finished with Vardy scoring then flagged. Then VAR can intervene. If the Lino flags straight away then either Vardy stops and/or the opposition stop and then there would be uproar.
But in this instance when it’s so clear, just raise the flag.