Question

We say energy can neither be created nor destroyed but it is interchangeable. And we know electricity is also a type of energy, so why do we say save electricity for future ?

Answer 1

Energy can niether be created or destroyed, but only be converted from one from to another. As per thermodynamics, there are grades of energy. Electricity, Mechanical energy, Chemical energy are considered to be high grades of energy. Heat is a low grade of energy.
It is possible to convert a high grade energy completely to a low grade energy, that is 100% efficient. While conversion of low grade energy to high grade energy is not efficient, and some of the low grade energy is rejected to the environment in the process.
So we convert the chemical energy of the fuel (coal or oil) into low grade heat, then into high grade mechanical energy and reject heat to the environment, then the mechanical energy into electric energy and again reject some heat.
When we say "Save Energy", we want to actually reduce consumption of high grade energy, so that we save our resources of fuel, water for a longer period.Also, this is like someone is generating using some resources and these resources may get exhausted.
So electricity is high grade energy. In simple words, we cannot convert 1kJ of heat to 1 KJ of electricity(considering finite environmental temperature) as some amount of heat needs to be rejected(see second law of Thermodynamics). Thus, wasting electricity is wasting money for generating it.