There are a few reasons why this would be useful:
- Spare card in case you lose one (esp when travelling).
- I can give a card to my wife so all expenditure can be analysed (I know, I know, connected accounts… but an additional card is simple fix for now)
- Use the additional card for all internet accounts - so if one loses ones main card, no need to update all payment information details on amazon, iTunes, Netflix, PayPal, etc, etc. The card could even be virtual, so it can never be lost
- Could be used to segregate expenditure (e.g. work-related expenses vs personal expenditure). (This could theoretically could be achieved with connected accounts, but that may add overhead transferring funds between them. )