You need calories for everything you do. They're nothing complicated - just a way of measuring how much energy there is in food. If you did nothing at all for a day you would still use some calories up just to keep ticking over. The more you move around, the more calories you use up.
The right types of fat in small amouts are essential for our health. Your body uses it to absorb vitamins, move nutriens around and build cells. However, because it tastes good, it's easy to eath too much. It's really easy for your body to store fat too, and some fats are just not good for us.