Multicellular organisms like humans are made up of cells of varying shapes and functions. Most cells are so small that they can see them only under a microscope because once they reach a certain size, they no longer grow.
Cells exchange substances through cell membranes to pass the gas(ex. oxygen) and nutrients necessary for life activities. In order to facilitate mass exchange in cells, it is more advantageous to increase the surface area by dividing into several smaller cells than to keep a single cell growing. Thus, multicellular organisms divide into two cells when the cell grows to some extent. This process is called cell division.