All organisms have to accomplish the same life processes. They need to grow. They reproduce. In order to do this they have to protect themselves from a hostile, changing environment, acquire and manipulate energy and nutrients, and get rid of waste products. They need to be responsive to their environment. For unicellular organisms, one cell does it all. Of course, that will limit the size of their accomplisments. One cell is not going to build the Taj Mahal by itself. That's going to take teamwork. And teamwork is the hallmark of a multi-cellular organism. They distribute the different chores to different tissues, organs and organ system, so that no one cell has to do it all Cells, tissues organs, and organ systems now get to specialize in ways that wouldn't be possible otherwise . For example, some organs can specialize in acquiring energy - like those of your digestive system, while some organs can specialize in getting rid of wastes - like your urinary system.