far and away, it's just by increasing fullness / filling-in for junky shit. there's many smaller reasons but they're not remotely as significant, no. that said, don't look at water as only something relative to your weight, as it is fuckin VITAL to health. being just a bit dehydrated is bad for your body and mind, if it's a chronic thing for you (better err on the side of too much than too little in this case, w/o a doubt)
ppl will definitely overstate how much water is needed in some cases; are you ever thirsty? can you correlate the sick feeling to anything?(i know that, for me, heavy water on empty stomach can be dizzying..)
the amount of water you need is really variable but is nothing more than a function of what you need for metabolism + what you lose through perspiration/urination, but it should never ever be some trivial afterthought, and if you actually feel thirsty you can be sure you're dehydrated! (don't force-feed yourself 8 glasses in a day if you don't need it - while minor-dehydration is farrr too common, ppl definitely overstate how much is needed, probably in a (shady)attempt to make sure those listening to them get sufficient water. you may not need as much as most tell you, but you DO need to not get dehydrated often!