Another reason people might buy a bottle of water is because of convenience - if someone's taking a long drive, for example, it's easier to buy a bottle of water at a gas station and drink it periodically than pull over every time they want to drink.
The reason I bring up soda is because there's sometimes less animosity towards soda than bottled water. Some schools will even ban bottled water, but allow bottled soda[1] - naturally leading to an increase in the less healthy option.
[1] http://grist.org/article/plastic-water-bottle-ban-leads-to-u...