Some people do not like paying with credit/debit cards but appreciate the ease of paying with cards, as opposed to bank transfers. You can offer a prepaid gift card to make online purchases easier for them, so that they only need to transfer money once in a while.You gain more liquidity if your customers prepay their gift card. You can get some free income if your customers pay for the gift card that will expire before they use it up, which often happens.
By offering one pre-paid card instead of asking your customers to purchase separate gift cards each time they run out of money on their card you make it easier for them to use the card (no need to save different bar codes, just one identifier). It could be also saved automatically on their account and they could be prompted to pay with it at the checkout to make it even easier for them.