(1) they are the "issuer" and thus get part of the fee the merchant pays to accept the card at transaction time.
(2) they have your money now so they can invest it and keep the interest earned between now and when you use the card. I know the interest on $100 doesn't sound like much, but if they get half a million cards out there, then they have 50 million to invest.
Answers & Comments
Verified answer
They make money in two ways:
(1) they are the "issuer" and thus get part of the fee the merchant pays to accept the card at transaction time.
(2) they have your money now so they can invest it and keep the interest earned between now and when you use the card. I know the interest on $100 doesn't sound like much, but if they get half a million cards out there, then they have 50 million to invest.
They charge you to put money on the card. Make sure you check for hidden charges like yearly fees etc.