We like cool water in hot summer days. And earthen pots keeps the water cool which is kept in it. We can say it is a good effect.
Now, we actually need to know is how it keeps the water cool.
The earthen pots have many minute pores present in it. Water kept inside the pot come to the surface of the pot through these pores and they evaporate absorbing energy. Since they absorb energy from the water and surrounding, it becomes cool. The cooling effect is due to evaporation.