When I was a young sprog (and yes I know that was eons ago), I was taught that the VRIAB was originally done for 2 reasons.
The first is that it is the fastest and most efficient method of getting a formation into the circuit and on the ground.
The second is that your aircraft is vulnerable to attack from another aircraft for a far shorter period of time and in an area where you may possibly have some form of ground-based air defence system, even if only mates with guns. This was - of course - at a time when enemy aircraft may well have been able to float around the skies in your area willy-nilly.