A big thing that people often forget is that cabin air is drier than what most people get in their day-to-day lives on the ground. That's why on long hauls on airlines that are good with service, someone at least goes down the aisle with the water bottle and cups every hour or so. It's easy to get a case of dry mouth or feel thirsty, and of course staying hydrated with the right beverages is good for you. And even if it's the wrong beverage and has a DE-hydrating effect, such as those often consumed on trips to Las Vegas, the airline is happy to get the revenue. I sometimes see people paying half what I paid for the fare on "happy hour" bundles on Frontier.

Usually Frontier doesn't roll the carts on the red-eyes if they don't think many people will order drinks. Unlike Spirit, they also have free water (though they don't publicize it). The legacies will give you a cup, ice and a soft drink (or other free item, like coffee).

I don't do many truly short hauls but I remember Delta having a tray with 3 choices (water, Coke, orange juice?) You could ask for something else and they'd try to get to it after passing out the trays and wouldn't guarantee they'd have time.

BTW, I'm a real fee-avoider on the ULCCs to the point of having gone to CUN and CTG on Spirit for 4 days each trip with just a rucksack, and I'll only pay for checked bags if I'm connecting off an international flight or have some other circumstance where I bought a lot in the destination. (Going as an air courier 25 years ago was a crash course in having to travel light). But I do often make an exception to get a $2.99 diet Coke if it's a 2-hour or longer flight. Especially if on F9 and I used status to get a stretch seat. It helps with the dry mouth and still seems almost like regular coach in the old days (and you always get the full can).