When people are sitting around making lists and deciding what they need to survive or just get by in the future there is one thing they need to give great thought to. Energy is the basis of everything. Energy takes many forms such as food, electricity, liquid fuels, mechanical, nuclear, solid fuels, solar and wind to name a few. Without it your body will not function, you will have no light, no heat, no transportation or communication. Energy is the foundation of everything humans do in this world. Without energy in its many forms we cannot live.
If you can produce enough energy you can do just about anything. You can live deep underground, live on the moon or even in space. You can heat your home, produce light to see, grow food, power vehicles and radios, produce goods from raw materials and power equipment to keep you alive.
Most people do not think about this very often because they can simply walk outside and feel the warm sun on their skin, do numerous tasks and even feed themselves as a result of this energy from the sun. They can do many things without modern technology because the sun provides the energy to do it. Knowing how to generate and utilize the various types of energy allows you to survive in many different environments.
Here is one example to think about to show you what I mean...
In the movie The Road, the earth is consumed by an environmental disaster that leaves the world in a cloudy haze, leaving it cold and dead. The survivors wonder around looking for any type of sustenance they can find including each other.
Even under the circumstances given, these people still have the ability to produce energy. Energy means life. Nuclear power would still work under these circumstances especially in navel vessels. Even though all of the trees seem to be dead, they can still be burned for fuel. As long as water falls from the sky hydroelectric power will still work. Where you have power generation you have the means to provide heat and light to grow food. You only need the knowledge and skills to do it.
In a survival situation, especially an extreme one, having an energy source that allows you to live is very important. That is one reason why knowledge and skills are so important to many preppers. Knowledge and skills allow you to construct a new living environment following an event and energy plays a major role in that. It is idiotic to walk around like the survivors in The Walking Dead, with no food, water, equipment or transportation when there are resources scattered all around you that can be utilized.
Simply burning wood can provide you with energy to run a vehicle, produce power...