The fluids - water, coffee, diet coke, tea, etc - keep you healthy while you lose weight. They help your kidneys deal with the high level of protein, they help your digestive track deal with the change in the material passing through you, to ease constipation. They counteract the diuretic effect of protein and prevent dehydration and all the dangers that implies.
It doesn't have to be water - but water is free, and safe, and easily available, even when you can't get your hands on tea of coffee, so it by far the best choice for most people to top up the intake to 2 litres a day.