From a school's point of view, they receive less money from the actual voucher, so either have to make the flight shorter or accept a cut in any profit, or even a loss.
However, they don't have to spend any money on marketing, and the voucher company will reach a much wider market since the same company is probably offering vouchers for flying lessons, track days, balloon flights and all sorts of other "experience days".
I think schools generally prefer selling their own vouchers, but clubs generally work on word of mouth, or having a good restaurant or a good airfield to fly into. The average guy on the street is probably completely unaware that he could learn to fly just down the road.