I used to think so too. But found out from that the water alone just didn't help the cramps i would get in my legs and feet at night. I went to the running world for help and was told, "you need to replace some minerals that you sweated out."
Water alone won't stop the cramps that i got from not replacing them.
I tried a few different things including gatorade, powerade, emergenC, bananas, etc.
I found that a tablet of magnesium does it for me.
Others swear by potassium or just salt tabs.
If it's hot, and you did some big climbs or had a sweaty day, and tend to cramp up at night, that's when you might need help replacing minerals.
I believe that's what the OP was asking.