It's been a long time since I worked in that space, but I think it is basically like a reloadable prepaid card you can get from visa or mastercard. I would assume there actually is a bank behind it, but the account is essentially being sponsored by someone else and there is less risk for the bank because you can't write a bad check or overdraw the account. That makes it potentially useful if the reason you didn't get an account is because the banks refused you or you couldn't afford the fees. For people who are just anti-bank or worried about financial privacy, they would still want to go cash only.

On a side note, reloadable cards can also be useful if you have friends or relatives that you want to help out now and then, especially if they are not local and maybe make poor decisions. It's cheaper than Western Union or a money order, more secure than mailing cash, and no risk of them having access to your bank account number from sending a check.