I've never heard of putting iodine on your skin to get tan.

I think part of it is to get a base tan before going on vacation so when your out all day in the sun you don't burn the first hour and then have a crap vacation for the rest of the week. But, yeah, there are people who use it as a make-up type thing. If it makes them happy, it's their decision. What I really don't like is seeing mothers slathering cancer causing sun screen on their young children all of the time.