Cells require energy to power their biochemical processes such as transcription and translation processes. The nutrients taken in by cells are broken down and the energy in these bonds is used to create ATPs, the energy currency of the cells.
During the breakdown of these nutrients for energy, in a process called cellular respiration, the byproducts produced need to be excreted from the cell. This is because the accumulation of these byproducts may be toxic to the cell.