As the water is forced through the kidneys, it will remove the excess sodium. Over weight people need more water than thin people. Larger people have larger metabolic loads. It also helps to prevent the sagging skin that usually follows weight loss. Shrinking cells are buoyed by water which plums the skin and leaves it clear, healthy and resilient.
Water helps rid the body of waste.
During weight loss, the body has more waste to get rid of - all that metabolized fat must be shed. Again . . . The colon is a primary internal water source. When the body gets too little water, it siphons what it needs from internal sources. Retained water shows up as excess weight.