...or if they have £2000 of yours then they know you plan to fly £2000 worth of flights with them and it is a way of keeping punters / forecasting financials.
The other side of that coin, though, is that if you've paid up front, the school has less incentive to keep you happy. In one London school, if ever a punter phoned up to make a booking, they would always bump people on pre-paid courses to make way for bookings which would bring in new money.