the purpose of Water Fasting is primarily to heal, the weight loss is incidental. Water Fasting gives the body time to heal naturally and cleanse itself - which results in weight loss since the body discards toxins.
there are many health benefits involved in water fasting, but to realise these, you must refeed correctly on raw fruits and salads and then maintain a healthy lifestyle.
personally, i do not agree with Water Fasting for weight loss unless the post-fast regimen is strictly adhered to. otherwise it is just like another crash diet which is detrimental to your health in the long term.