[waste_monk]

Well, they used to treat diarrhoea by withholding fluids as an attempt to "starve it out", maybe it's something along those lines. E.g. it makes you heavier, you want to drink as little water as possible to stay light, or to minimise bathroom stops, or something like that.

[buran77]

Whenever I was doing intense physical effort drinking water made me feel sick. When I was a teen I didn't understand why so I assumed water is bad right after exercise. But later on I started reading on this and using some common sense and I started putting salts/minerals and some sugar in the water I was drinking and my body reacted completely differently. This was before sports drinks were a thing.