Some news stories are saying that we should only drink water when we actively feeling very thirsty. While they are well intentioned, this is like saying you should only deposit checks in your bank account when your ATM says "balance zero".
The human body needs water for practically every single thing it does. Your body is about 60% water. Your brain alone is 70% water. Your blood is primarily water and it needs a proper level to carry nutrients throughout your system. It cannot work with large doses of water during a meal and then no water at all during the intervening hours.
Water helps to flush your body of toxins and impurities. You don't want all of those toxins just sitting in your kidneys and liver all day long! You want a regular, full amount of water moving through your system, coming in clean, going out with the pollutants, regularly through the day. That keeps your system as clean as possible.
How about weight loss? The more you stay hydrated, the better. Water helps to carry fat cells away from their storage location, and to flush them out of your system. You need water so that the fat is metabolized properly, and so the leftover items are removed. It is very important for weight loss that you drink ample water.
Why do people say you have "water gain" or "water loss" when you are on some diets? Your body always needs water in it. Some diets try to get you to dehydrate - and then claim you are lighter because that water is gone. On one hand that is technically true - but on the other hand you are now unhealthy! That would be like whittling out your bones and claiming you are lighter because you have empty, hollow, fragile bones. You need your bones! You need your water too. When you lose weight you want to lose FAT - not bones or water.
On one hand I agree with the *principle* that you should be drinking because your body needs water. But on the other hand, I think we have enough proof around us that modern bodies are notoriously bad at hearing signals about "I am thirsty" and "I am full". We keep eating even though we are technically full. We don't drink even though our body is thirsty. Sometimes we eat more sugary snacks even though what we are is thirsty! So the way to retrain your body to be healthy is to keep water available, and to drink it regularly. Pay attention to your body's signals. This will help you become healthy and happy long term!
Lisa Shea's Library of Low Carb Books