Fat Is A Great Source of Energy:
One of the many complexities about lipids is that they digest slowly in the body. Thus, if you eat a meal heavy in fats, they stay in the stomach longer and you feel "full". Eat too much and you start a nice collection of fat in the liver, metabolic system and in various storage points around your body. (Usually these storage places are where you don't want fat to be stored).
Fat also is a rapid source of energy -- but only if you work at it.