You are what you eat. If you eat junk, it will be reflected in your health. Your body will lack proper nutrition, you will be sick more often, you will age faster and not look as good, and you will likely have many more visits with your doctor. If you don't exercise, you will lose muscle mass and calcium density in your bones. In addition you are much more prone to injury and will have a lack of energy.
The top 10 causes of illness and death in the US have a nutrition component and most also have a lack of exercise component. Maintaining a healthy weight and getting regular exercise (at least 30 minutes per day, 5 days per week), can prevent many illnesses. If you want to live a healthier life, make an investment that absolutely positively pays off; eat healthy foods and exercise regularly.