Burning fuels such as hydrocarbons is useful because it releases light and heat energy. This process, called combustion, involves hydrocarbons reacting with oxygen to produce carbon dioxide, water, and energy. The energy released is primarily thermal energy (heat), and also light energy, both of which are useful for heating, cooking, and powering engines. The reason energy is released is that the chemical bonds in carbon dioxide and water formed during combustion are stronger and more stable than those in the original hydrocarbon and oxygen molecules. The difference in bond energies results in the release of energy, making combustion an exothermic process.

In summary, burning hydrocarbons releases light and heat energy, which can be directly used as thermal energy or converted into mechanical energy in engines and power plants. This energy release is what makes hydrocarbons valuable as fuels.