Differences in energy storage are important factors in mobility of organisms. Plants store energy as starch, while animals use fats.
Fats are lower density than starches, and so weigh less for a given amount of energy. Due to this, fats are used in mobile organisms, as weight is of great concern. Starches have a lower energy density, but the mass is less of a concern to plants. This is because their immobility means they do not need to be able to move themselves.