One of the latest methods of entry for vans is to attack the original factory-fitted locks. These are easily opened if you know where to attack them. The shields are necessary when you have deadlocks for 2 reasons:
1- If you’ve locked your central locking, but don’t have your deadlocks locked for any reason, (maybe you’re working on site somewhere in a ‘safe’ area, your labourer forgot to lock them after getting something from the van, etc), the shields will protect your original van locks from being bypassed.
2- if your deadlocks are on, the deadlock will be the only thing holding it closed, when the door latch is opened. This means the thieves the free to bend the door upwards and get into your van underneath.
The bottom line is, the original door locks are very strong if they can’t be disarmed, and the shields are an inexpensive way of giving yourself an extra layer of security all the time.