When you go to the grocery store, you find that the cheapest calories are the ones that are going to make you the fattest - the added sugars and fats in processed foods.
It's really important for your health, because you will never use as much salt and fat and sugar as a corporation will use cooking for you.
One of the reasons we eat fast food is that we don't have to cook fast food. We are out-sourcing cooking to corporations, they tend to cook with far too much salt, fat, and sugar.