The idea here is not about crediting (lending money from Curve), but just about issuing a credit card instead of a debit card.
There are some situations when you can’t use your Curve card: e.g. some car-rental companies require a credit card for a deposit. Also I’ve recently had a flight with Level — and it’s not possible to pay during the flight with debit card there, you can use only credit cards.
In addition, afaik, fees on credit card transactions are a bit bigger than on debit cards