If you have a recurring charge, you make the sale once. If it's charge on demand, then every time the customer uses it, he thinks about the cost. Furthermore,

If you have a recurring charge, you make the sale once. If it's charge on demand, then every time the customer uses it, he thinks about the cost. Furthermore, $X per month is usually a lot less than $Y per use, per charge. All and all, a much easier sale.