The Second Law of Thermodynamics implies it’s not wise to convert high-grade energy, such as the chemical energy in a fuel, into lower-grade thermal energy until the latter is needed. The chemical energy in most fuels can be stored for years. However, once that energy is converted into heat, it’s a race against time to contain it.
The insulation of the container and temperature difference between the heated material in the container and the materials outside the insulation system determines the pace of the race. In the end, higher temperature always yields to lower temperature and the usefulness of the original energy continually diminishes.