The refrigerator works more efficiently when it is fuller. Because water holds its temperature well, storing a container of water in the refrigerator helps it regulate its temperature. This also provides you with a regular source of cold, refreshing water throughout the summer, eliminating the need to run water from the tap while waiting for it to run cold. In addition, leaving the water container uncovered in the fridge allows the chlorine to evaporate out and will taste fresher.
See more information at the Department of Energy.