Is cleansing all it's cracked up to be?

Maybe I've been brainwashed over the years but how in the world can fasting and cleansing be good for you? Every "expert" I've ever known has told me that if you let your body go for hours without feeding it, then you slow down the metabolism and it goes into "starvation" mode. So the next time you eat, your body stores the food as fat for the next time you are in starvation mode.

So how can cleansing be good for you?