Life uses Energy
We have established that all living critters have several levels of organization. Since you continually reorganize your impeccably kept biology notebook as if you were a squirrel hopped up on caffeine, we know you have no doubt that maintaining organization requires energy—lots and lots of energy.
Think of all the calendars, day planners, and lists that you have made in your lifetime (or haven't made, if you aren't the planning type). Organization is hard! Cells acquire matter and energy to not only maintain their structures and functions, but also to grow and reproduce. Bow chicka wow wow. The matter and energy acquired by a living organism are used in chemical reactions, and the sum of all these chemical reactions in one living organism is referred to as its metabolism.
Living organisms get their matter and energy from the environment. Depending on the type of organism, matter in the form of nutrients can be "sipped" from air, soil, water, and/or food. Energy, on the other hand, is created either by photosynthesis or by extracting it from other living organisms. That would explain why little ol' humans like us feel mighty sluggish after going a few hours without eating.
Some living creatures—like plants and many unicellular organisms—perform photosynthesis: they convert light from the sun directly into usable energy. Other living things—like fungi, non-photosynthetic unicellular organisms, and animals—eat other living things. Not very neighborly, but hey, it’s a dog-eat-dog-food world…