It doesn't have a "magic" purpose in losing weight as people say. It allows the body to function more efficiently, which leads to overall greater performance from organs, blood etc etc. It can be used to keep you occupied instead of eating more food, but that's about it as far as weight loss goes. As in anything that you could notice without testing.
It's calories in vs calories out for weight loss, water is just a tool used to maintain a greater state of health, which is most important.