In our case it's a matter of convenience.

When at home we drink tap water; when we're on the go we throw a couple of bottles of cold water in the car. On very hot summer days I sometimes give a cold bottle of water to the fellows driving the garbage truck; until the move to community mail boxes, I would leave chilled bottled water at our front door for our letter carrier ( who was most appreciative).

One of the main reasons that we keep bottled water on hand is that it helps to curb the urge to reach for a sugary carbonated drink ...