Why are there water stores?!

I see quite a few of them, as well as people lugging the big plastic water jugs to them to get filled up. I understand people might want cleaner water to drink and cook with aside from the tap water, but it seems odd to have a dedicated store just for that. And I've seen 3 different ones open in my area within a couple years and they seem to be quite popular. Basically it's making me wonder where the huge demand for filtered water is coming from. Maybe it's because I never thought the tap water was really that bad.