Probably a stupid question, but that just means it'll get answered faster.
During peak hours, there is more demand than what they can supply. People get home, they cook, the fans and aircon's (or heaters) gets switched on, they shower after a day at work (new water that needs to be heated in Geyser), lights go on, etc.
They are not able to store electricity, so it's not like they can save some up at certain times, like for instance at night, for use later. So there is no use turning of the electricity if they can supply whatever the demand is.
During the day there may be load shedding (even though the demand isn't as much as it normally is during peak hours), but then they do maintenance (planned or unplanned), so at that specific time, the supply is then still less than the demand.
TL;DR: At night people sleep, so there isn't much electricity use and they can meet that demand.