Plain water can be fine for hydration depending on your other fueling, but there are two use cases where I really like to have these tablets added to one of my bottles:
1. On rides over an hour, but less than two, I often find that a bottle of water with a Nuun tablet is sufficient fueling, and I don't need anything else (gel, bar, etc.).
2. On very hot days, water with Nuun seems to go a little farther than just plain water for keeping up with hydration.
I wouldn't necessarily seek out the flavor off the bike, but it's light enough to be inoffensive, and I do think that in addition the electrolytes and caffeine there is some psychological boost to drinking flavored water and feeling more satisfied than with plain water.
I have used these a number of times in the hot humid summer and have performed well and felt relatively good after the rides. Granted I have no real idea if it was a result of the supplement or "dumb luck" . They do dissolve well and have a good if muted taste
I mostly drink only water while riding the bike, but for times when it's hot, Nunn's flavor is subtle, and easy to drink. Caffeine dose is small, so it's only barely noticeable, but it does help keep you moving late in the day.
I ride in a very warm and often humid climate. Rides of 40 miles or more, especially with lots of climbing, can be a recipe for cramping so supplementing with electrolytes is a good preventative measure. I've tried Hammer Products as well but I like these better because they have caffeine and a shot of B vitamins. I personally like the light boost from the caffeine on a long ride. The taste of this stuff is not too overpowering and there's no strong, fake sweetener, which can be hard to take when you're doing hard efforts. I use this stuff in at least one of my two bottles for all my rides of more than 30 or 40 miles. I also distance paddle outrigger canoe and SUP on the ocean and it works great for that too.