Fuel

Life as we know it is a kind of repeating pattern that develops among (mostly Carbon-based) impurities in liquid water as a result of energy flow through that aequeous medium. The necessary energy flux may be provided by solar radiation, or by geothermal vents in the sea floor, or by exothermic chemical reactions of various kinds (including eg the dilution and interaction of concentrated salts extruded at the geothermal vents). Since a by-product of the solar powered organism-building process is oxygen, several billion years of such activity have created an oxygen rich environment in which oxidation is perhaps the most readily accessible exothermic reaction, and this has become the exclusive source of energy for the life-processes of many organisms such as humans and other animals - which all survive only by virtue of the oxidation, within their bodies, of fuel provided by the ingestion of other living things consumed as "food".
Humans also enhance their activity by harnessing energy outside their bodies. The first, and still predominant, mechanism for this is by combustion of fuel (most often in the form of the dead bodies of other living things, such as plants in the form of wood and coal, marine organisms in the form of oil, and the decay products of both in the form of methane). Other power sources include water flow, wind, geothermal gradients, solar heating, photo-electric, and nuclear reactions - but combustion of carbon compounds remains the primary source, and this now adds a significant amount of CO2 to the atmosphere which creates a risk of long term environmental changes which may well be very harmful to humans.

Although some energy-consuming processes such as milling grain or smelting ores can be carried out at a limited number of fixed locations, most purposes (transport, cooking, home heating and entertainment devices) require the consumption of energy at a variety of locations so the matter of energy transport is very important. Power lines can transmit electrical energy over long distances but only to specified target locations. It can also be carried in batteries (and capacitors) but their capacity is still quite limited. So we seem to need some kind of energy intense portable medium - ie fuel. (Actually batteries might be considered as carrying a kind of fuel, even though its energy is not usually extracted via combustion with released exhaust, and they, or fuel cells may indeed be the way of the future but they are not yet ready to meet all of our current demand.) Nuclear fuels can be transported (though some consider the risks to be high), but the scale of an efficient reactor is so great as to make it usually immobile. Manufactured Hydrogen for combustion, or fuel cells and batteries could meet our needs without putting CO2 into the atmosphere but carbon-based chemical fuels extracted from the ground still seem to provide the most economically attractive alternative.