THERE'S JUNK IN JUNK FOOD

In college I was a bartender. One of the more popular request was rum and diet coke. Even then it made no sense to me why someone would be concerned with ordering a diet coke with their rum. Much in the same way I find it a bit odd that people will at junk food and take a handful of vitamins. I'm not certain if that makes a difference. I suppose some would argue that taking the vitamins would at least be helpful. Still I wonder if one is really concerned with being healthy--why eat the junk food in the first place?