[hiyer]

What's the advantage of this for hydration when compared to plain water?

[Cockbrand]

It's got what plants crave. It's got electrolytes. - well, at least the electrolytes part is true. When you sweat heavily, you lose minerals and salts, which can become detrimental to performance or even dangerous to your health during intense workouts. Hydration products [claim to] replenish these.

[kennu]

Usually plain water is fine but sometimes you need a bit of extra electrolytes to rehydrate. I wouldn't drink too much of them though.