1. A cell must work just to maintain its complex structure, for order is intrinsically unstable. Because of entropy (the second law of thermodynamics), which states that any enclosed system tends toward disorganization, cells must constantly use energy to keep themselves in order. If the cell was to stop using energy and let itself go then it would eventually get so disorganized that it wouldn't function properly.
2. With the help of enzymes, the cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy. An example of this is gasoline; before you burn gasoline in the cylinder, it has a lot of energy and high potential