Multicellular organisms are made up of millions and trillions of cells. All these cells perform specific functions. All the cells specialised for performing similar functions are grouped together as tissues in the body. Hence, a particular function is carried out by a group of cells at a definite place in the body. Similarly, different functions are carried out by different groups of cells in an organism and this is known as division of labour in multicellular organisms.
In multicellular organisms, cells might have seperate jobs or tasks. In a unicellular organism, the only cell present will have to complete all tasks neccessary for survival.
With multicelled organisms, it allows for certain cells to specialize in their function, and divide the labor between multiple different cell groups.
In the human body, our skin cells provide a certain task. The cells in the lining of our intestine also provide a certain function. Now, these do not provide the same function, they each have their own jobs to do. By dividing labor, multi celled organisms are able to complete more complex tasks.