The thing is that GOG wants to be sure that they get their money, i.e. that your bank account contains enough positive numbers. And they want to know instantly. A direct debit cannot ensure this, as the bank acount could be empty. So instead of negotiating with your bank directly, they get their money from paypal and credit card companies while paying them a fee. Now paypal and credit card companies deal with getting the money from you. In principle, its not neccessary, but because instant international direct transfer isn't just available yet, ... we still have to wait for it. But I guess, that killing the middlemen or installing more competition would surely make the whole process more efficient.
Actually outside the english speaking countries, credit cards aren't so popular. I don't use my credit card here in germany, an EC (direct debit) card is enough, and some online shops even send you an old fashioned bill. :) I just have it, because outside of germany sometimes it's the only possibility and it's for free.
