Metabolism is important because it is literally the powerhouse of the body, providing energy to keep the body going. In fact, many science and biology dictionaries describe metabolism as a process which is necessary to sustain life. Without metabolism, living organisms will die, and errors in metabolic processes can cause health problems such as diabetes, in which the body fails to metabolize blood sugar properly.
Living organisms are in a constant state of flux. To do anything, from firing a neuron to alert the brain that smoke is in the air to generating an extra burst of power to pull ahead in a foot race, the body needs energy. This energy is provided through metabolism, in which the body breaks down the substances ingested, and rebuilds them into useful substances, including raw energy and components which can be used to transport energy from place to place.
Likening metabolism to a powerhouse is very accurate, because this process involves the generation, storage, and transmission of power, and like an electrical grid, the body is very vulnerable to metabolic imbalances. For example, if someone metabolizes food too quickly, he or she tends to remain very thin, because the body cannot store energy in fats and muscle. Conversely, people who metabolize slowly may not be able to access the energy they need, because their bodies may not have generated it yet.
Some people have genetic conditions which cause problems with their metabolisms. These inborn errors of metabolism can include things like lacking enzymes which are necessary to break down food, and they often require medical intervention to be corrected. Metabolic problems can also be acquired, as in the case of someone who develops diabetes late in life, or in the case of someone with an eating disorder who causes permanent damage to the metabolism through consistent starvation.
One of the most common reasons to explore the metabolism is because someone is trying to build up strength for athletics, or to lose weight. Understanding how the metabolism works is critical for both of these tasks, as people can engage in activities which will support the metabolism to accomplish the desired task, or they can undermine their metabolic processes, making it harder.
Everyone's "powerhouse" is slightly different, which is one reason why there can be a lot of physical diversity between people who have similar diet and exercise habits. Finding one's own metabolic rates can be valuable for maintaining general health, as one can make lifestyle adjustments to cater to the specifics of the body.