Storing excess glucose as glycogen allows for energy to be stored closer to the site of use (in muscles), and is able to be quickly used. This is important during long periods of movement, such as long distance running, providing an energy source the can be mobilised when glucose supplies are exhausted (after around 1.5 hours of activity).
Fat is insoluble, and cannot be converted back into glucose. Instead, fats are broken down by beta-oxidation, which can then be fed into the Krebs cycle. Conversion of glucose to pyruvate and then fat (by a fatty acid synthetase) allows the storage of energy for longer periods of time, reducing the amount of energy that is lost by an organism. Fat can also be beneficial to the survival of an organism, reducing heat loss and allowing an animal to continue to be mobile (as fat has a low density).