According to the cell theory, cell is the basic unit of life and all living things are composed of cells. Cells are capable of carrying out all the life processes, though maybe not as complex as higher organisms. All of our bodily functions are carried out by cells. Human beings are complex organisms and require complex systems (such as circulatory, respiratory, reproductive, etc.) to carry out the tasks required for maintenance and growth of human body. Each of these complex systems is composed of organs, which are made up of tissues, which in themselves are made up of cells performing similar functions. Think about our nervous system, which is made up (ultimately) of nerve cells. Cells are also important for our environment. An example is the process of cellular respiration, during which cells break down glucose and generate carbon dioxide and ATP molecules. Through this process, cells generate carbon dioxide, which is a necessary ingredient of photosynthesis (the process which produces food and oxygen for us). Carbon dioxide also causes greenhouse effect and ensure the warm temperature on Earth.
A number of other examples can also be thought of.
Hope this helps.