Fasting is an exceptionally ancient, and powerful, approach to healing many common disease conditions. It allows the body to rest, detoxify, and to heal. During fasting the body moves into the same kind of detoxification cycle that it normally enters during sleep.
Tip: Any practice that promises to flush "toxins" out of your system is more than likely based on nothing but baseless pseudoscience. Ask proponents what sort of "toxins" they are talking about, and they will look confused. It's like asking Deepak Chopra how his beliefs have anything to do with quantum mechanics, and you discover that he just uses the word to sound more legitimate and less obnoxiously stupid.

A lot of "alternative" medicine has been used for thousands of years. But it is called "alternative" because it doesn't work. Fasting is not healthy and definitely not a good way to lose weight.