1) some customers want an appointment without paying at that time. Obviously, you prefer the opposite. However, if a customer might switch providers based on having to pre-pay, you might prefer to take the booking.
2) if the two systems are linked, there is the possibility that prepay will fail and you might lose the booking. This can happen, and the customer is sometimes unaware of the failed booking/payment and then shows up for an appointment that has since been properly booked.