quietlystructural answered your question: Why do people buy bottled water?
Because they lacked the foresight to bring a refillable bottle with them?
mountland answered your question: Why do people buy bottled water?
Easy to carry around, campsites don’t have sanitised water so have to use chlorine tablets which taste awful so use bottle for drinking water
I understand that. I've bought bottled water a few times (especially when I'm working) because I forgot to bring my own. I was really talking about the people who only drink from bottled water.
truthinsilence answered your question: Why do people buy bottled water?
Due to the lack of good filtration on public water
Ok, this is such a myth that the bottled water companies perpetuate so that people will buy their water. Unless the bottle says reverse osmosis (RO) or distilled (DI), bottled water is just tap water run through a carbon filter* (which, don't get me wrong, there's nothing against carbon filters, but it's really inexpensive and so bottled water is way overpriced for that).
Bottled water companies want you to think that their water is so much purer, safer, cleaner than tap water. Actually, bottled water is full of dangerous chemicals. A test was performed where seven brands of bottled water were sent to be tested by two different labs (TestAmerica and Dr. Michael Summer, an environmental chemist). The results were reviewed by Dr. Stephen King, an epidemiologist and toxicologist of Toxicology Inc. Both tests found toluene (a neuro-toxin linked to cancer that is found in gasoline and paint thinners), styrene (a cancer-causing agent), three different types of phthalates (which can cause dysfunction, especially in the fetus), and all three can cause adverse reproductive outcomes. Larger jugs of bottled water are made out of Bisphenol A (BPA), which is water-soluble (so it can easily leach into the water that the plastic is carrying). It's an endocrine disrupter and can mimic the body's own hormones, which can cause extreme health problems.
The thing is, tap water is highly regulated, but bottled water is virtually unregulated. FDA can only regulate interstate commerce (meaning that they regulate products that travel across state lines), but 60-70% of bottled water is produced and consumed within state lines, so it's not under FDA regulations. Only one person in FDA is responsible for all of the regulation of bottled water in the US (because the FDA is so overwhelmed with everything else they have to do). Bottled water manufacturers are supposed to do their own testing on their products and sources, but they are not required to submit a report to the FDA. Bottled water from Nestle, Perrier, Coke, etc. have been recalled many times. However, tap water is constantly being regulated (many many times a day) by the EPA and by municipalities. In a city over 1,000,000 it has to be tested 300 times a month, in a city over 3,000,000 it has to be tested 400 times a month.
If you're still really worried about impure tap water, buy a Brita filter or something, which only cost about $30 and the filters cost about $20 (which only need to be replaced about every 2 months). Bottled water, however, costs more than gas per gallon, but no one complains about that.
I don't know, I'd rather drink tap water any day.
(All of this information comes from the movie Tapped, except for this* which comes from here)