Many studies over years have shown benefits of drinking water. In reference to losing weight, greatest benefit is positive affect water has on your metabolic rate.
Water promotes a thermogenic state, which in return increases your metabolic rate. The faster your metabolism, slower your propensity to gain weight.
But is it really water that I am talking about? No. What I am really referring to is hydration. The human body works hard at keeping a certain amount of water and salt in your tissues relative to amount of water and salt in your blood.
Proper hydration is a reflection of correct balance of water and salt that exists between your blood and surrounding tissues. Your diet should help your body sustain correct amount of water and salt in your blood and not disrupt existing balance between your blood and tissues.
Let me explain...
Water & Salt Balance
True hydration is sustaining an amount of water in your blood relative to your tissues that reflects overall water and salt balance your body works at creating. Looking at graphic, fluids that cause water to move out of your blood or into your blood are not ideally hydrating.
Salt Moves Water
For a fluid that you drink to be hydrating it should have little effect on moving water. Water will always move to environment with higher amount of salt. Looking again at three possible "tonicity" states, which one is most hydrating?
Hypotonic Fluid -- This dilutes your blood and now surrounding tissues have more salts. Water moves out of your blood and into tissues where greatest amount of salt resides. This re-establishes water/salt balance.