Its quite dangerous to drink lots of water as the body needs salt and other electrolytes. I guess that's why the pure water would be a problem. Potassium and sodium are finely balanced by the body so can easily be knocked out by illness or deficiency, (hyponatraemia (sodium) and hypokalaemia (potassium) and conversely when you have too much (hypernatremia and hyperkalaemia), especially as you get older too. Athletes tend to drink electrolyte drinks to replace those lost whilst they exercise. The body is amazing, its scary when it starts to go wrong.Ah okay I guess it would be to do with things like sugar or protein in the drink slowing down the absorption. It's mostly sodium and potassium that would make a drink more hydrating but need to be careful with things like that as well especially in the heat and or if someone is training hard
For example I just found out the recently popular Prime drinks are marketed as hydration beverages but actually contain very little sodium to be that.
Also heard of a case of a guy who played tennis in the heat and was hydrating himself with coconut water, which is high in potassium and almost died