Every living organism needs the food for the same reason that we do. We need food for energy and as a source of nutrients. The breakdown of food, through cellular respiration, releases energy in the form of adenosine triphosphate (ATP) molecules. Energy is required for a large number of body functions, including walking, talking, sleeping, running, eating, watching television, reading, etc. (the list can be very long, just think about whatever we do throughout our life). Food is the only source of that energy. Apart from working as a food source, it also provides us with nutrients, which are also used for synthesis of biomass (think about muscles or how the cuts and bruises heal over time, etc.) and other molecules necessary for our life. The partially metabolized products of our food may also be useful in the anabolic activities, from which we gain body mass and muscles, etc.
Hope this helps.