Waterbug, I used to believe the same before, just a couple months ago. But then I started to read in order to educate myself more in the subject of fasting. I found out I was wrong. Now I believe that drinking water without thirst while fasting dilutes electrolites the body needs to function properly during that period of time.
I have been reading about homeostasis or that mechanism that push our bodies to always try to maintain perfect balance or harmony within.
We just have to read a little to learn many things. Fasting is not talked about in depth in modern schools. But the subject is ancient and many have studied it before.