Apparently it's supposed to be a diuretic or laxative (not much of one though...)
Back in the late 1970's, I was going to Diet Center (anyone remember them...? You were required to weigh in 6 days a week except on Sundays when they were closed...
gimme a break I was only 15 at the time...). Anyway, one of the things I remember was that instead of breakfast we were supposed to have a cup of hot water with the juice of 1/2 lemon added (along with a potassium supplement, which was sold by the Diet Center, of course!).
Right now I think it's being peddled as part of the current "detox your way to weight loss" fad that seems to be making a comeback - if there's one thing I've learned about diet fads, it's that after awhile, like a bad penny, they seem to return on the scene. At this point, I'm waiting for the Beverly Hills Diet to make a big comeback and anticipating the questions that are sure to pop up about Papaya Day.