It's not really a challenge, although I'm sure there is something to be learned by fasting. There are lots of religions (almost all in fact) that have a fasting component, whether to achieve enlightenment or to learn discipline. I'm not primarily interested in all that though it's a great benefit.
The purpose of this particular fast is to cleanse your body of old toxins, excess mucus stored in joints and muscles, to give your digestive system and organs a chance to rest (as opposed to spend energy eliminating toxins), and to clean out the intestinal tract. This is all meant to increase vitality and improve health.
The holistic viewpoint is that disease is an accumulation of toxins and mucus in your body, and that if you don't do something to flush it out, then you'll become more sick.
I'm not overly sick, per se, but I do have allergies and my joints could use some loosening up, and I'm sure that my digestive tract has dren (pun intended?) inside of it that needs to move.
More information can be had at:
www.frenzy.com/~sam/mc/TheMasterCleanse.pdf