Other measures are also helpful but we don't generally think about them. For example, carefully counting "water-in, water-out" (WIWO) seems like it'd be necessary to avoid dehydration. However in normal circumstances (i.e., when you're not running marathons or exerting yourself in the desert) we don't need to think about this very much. Your body will manage its own hydration levels by making you thirsty only at the appropriate times. WIWO is still a thing but we don't obsess about it.

Now imagine your body couldn't manage its own hydration properly, perhaps because we contaminated our water supplies with something that disregulates the thirst response. Then you'd have a society that spends a lot of time talking about WIWO and it'd be a pretty sick place to live. It's much healthier to treat the underlying disregulation than to force people to constantly be thirsty.