Question

On a hot sunny day, why do people sprinkle water on the roof or open ground

Answer 1

because as the water evaporates it takes way the heat inside of the room. 

Answer 2

When people sprinkle water on hot and dry roof on a sunny day, that water evaporates by taking energy from the ground, roof and surroundings. The evaporation of water causes a cooling effect because the large latent heat of vaporization of water helps to cool the hot surface.