This is nothing to do with prepayment.
For those who have been living under a rock, we don't have the energy capacity we need when demand is high due to winter, not helped by the wind turbine I can see out of my window not spinning at all today.
If they reduce demand at peak times, it reduces the need to use polluting old coal fired power stations kept on standby to cover gaps in generating capacity over winter.
People are getting a small financial incentive to smooth out their energy usage, to reduce peak demand. But many aren't motivated by the money, they like the personal challenge, to do their bit etc.