to cleanse the body of toxins, show your appreciation for those who have nothing so you can feel a little of what they feel, or have the feeling of giving up something that your body so desires....I'm sure there are more reasons than just those 3..but from a lot of perspectives, those are the main ones.

As far as not knowing how people can go for that long without eating or drinking..basically its just a reversal of the bodily clock...instead of not eating during the sleep cycle, you eat...and not eat during the waking cycle. I'd be more wary of the people not getting enough sleep than I would be of not getting enough to eat so they can think during work..
I think the idea behind Ramadan originally is wonderful...

Grant me the senility to forget the people I never liked,the good fortune to run into the ones I do,and the eyesight to tell the difference.