Why eat junk food if it is bad for you?

Junk food is not very nutritious for you, and often contain ingredients that are harmful to your body. Such as trans fat from partially hydrogenated oil, because it's cheaper to mass produce than normal butter. They are typically much higher in calories as well, which will lead to weight gain over time. They are also lower in fiber, which can cause gastrointestinal problems. The fats in junk food are also not the fats your body needs.

If you're curious about the harmful effects of long term fast food intake, look at a documentary called "Super Size Me".