Firstly, it rehydrates your mouth, which allows your salivary glands to ramp up production and wash away any bad taste from the reactions that can happen when your mouth is very dry. Secondly, it's something your body needs, so it creates a pleasurable sensation anyway.
Daylight water takes on a flat taste. Most people aren't even aware of the change, but it's there none the less. Then at night, when water sits in a darkened room, the absorbed daylight ions are displaced by heavier night time ions and water takes on a cooler, more refreshing taste.