Mostly bottled water is waste but in some places it is prudent to buy some to keep water in stock at the house for emergencies.
Some people are probably also trying to drink more healthily and bottled water is easier to think of as a replacement beverage when trying to reduce on sodas and sugar drinks, it is also easier for taking along on the bus and the day.
Personally I can keep up with carrying my own tap water, but again, a lot of people on SNAP are not the most disciplined and detail oriented among us.
“Some people are probably also trying to drink more healthily”
Or they are just lazy, and spending other people’s money carelessly. If they were using the money to stock up on bottled water for disaster prep, I’d be fine with it. But you and I know that is not what is happening. Like 99.9999% of that is not that.