The study focused on Muslims observing Ramadan, which does forbid drinking liquids (including water) during the daytime.

“Dry fasting” is a thing, but it seems most people recommend against it. Not sure what the added advantage is for avoiding zero calorie water consumption.

Pleasing god. The high you might get from knowing you have the amount of self control necessary to dry fast.

I mean, there's the religious "high". But I think the goal behind it is to allow one to feel how those who are much more unfortunate than us feel on a hot summer's day and so on.

I'm not sure how things used to be, but having lived in a Muslim country a decade ago, my experience was during Ramadan daily life carried on as usual with people going to work and school. Instances of car accidents (often fatal) during this time were much higher than other times of the year.