Why do many say it is not good to water planets , flowers when it is very sunny?

One reason is due to evaporation, its the most inefficient time to water. Another is the water droplets can actually act like little magnifying glasses, and burn the leaves, for delicate plants anyway, if its very hot, like here in the california valleys. As a farmer, I must water during the day as well as night, but I have no choice, but those are orchard crops, and the water never hits the foliage. For the stuff around the house, I set it for early morning.