How does the 7 day free insurance work?

Norwich Union offer free 7 days insurance when you buy a your car, so you can leave the show room or what ever. But whats the catch? And why would they offer it for free, they always say statistically new drivers are likely to crash and all... So whats the idea of the free 7 day insurance and whats the catch?
Update: Sorry, but im having a blonde moment, can someone explain it more please?
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